How should I choose the right pick thickness and what are the differences between them? I'm using a 0.5 m pick for extra light strings, but I don' t like it very much.
What is the best pick thickness for strumming ?
How should I choose the right pick thickness...?
The best way to choose the right pick for you (since it's all about personal preference and your particular playing style and guitar setup) is to buy a variety of single picks and try them out.
Go to a music store and ask about single picks. They often have a plastic box with dividers and a huge variety of picks organized for not much money per pick. Don't just buy random picks (well you could, but I think there's a more intelligent way to find the best pick). I would stick to the traditional pick shape. Buy a few different materials and/or manufacturers, and buy a range of thicknesses for each material/manufacturer.
For example, you might (or even should) try the Fender celluloid picks, but don't just buy a random Fender celluloid pick, instead, buy a range of thicknesses or buy one of each thickness. Thin, medium, and thick are probably the most popular ones. Extra thick is available but is pretty extreme.
Dunlop nylon picks are also very popular, and are available in six different "guages" (labelled by milimeters of thickness). Pick a style and buy one of each. Look for other options and perhaps spring for a few unusual picks if something catches your eye. It often helps to know what you don't like when trying to find what you do like.
When you get all the picks home, spend some time playing with each different kind. If you realize you like a material, stick with that for a bit and try different thicknesses, and/or vice-versa. You don't have to find the perfect pick on your first time out, but you will probably at least have a favorite one out of the selection you purchased. That's your new baseline, and you can try out a few other picks as time goes on and either make a change or stick with your first favorite. You'll probably know it when you find the one.
...and what are the differences between them?
Assuming we only talk about the standard shaped picks, the material and the thickness control the overall stiffness of the pick. That's the primary difference between different picks.
Picks that are more stiff (thicker, harder material):
Picks that are less stiff (thinner, softer material):
The shape of the pick does matter. One advantage with the traditional pick shape is it actually lets you use different shapes all in one pick. Normally people play with the pointy end, but you can flip the pick around and either play with the rounded end, or you can play with one of the corners of the rounded end (the last one is how Eric Johnson uses a traditional shaped pick). If you learn to use the traditional shape, it will give you the best place to start when selecting other picks.
Pick style and thickness and material are largely a matter of personal preference. You should try many different type picks to see what works best for you. As your playing skills evolve, your choice of pick may change.
I will talk about different type picks but also want to touch on some ideas on how you might use a pick as part of your strumming technique because the way you hold the pick and the way you engage it with the strings will make a big difference in the overall level of satisfaction you experience with any pick.
Most beginners find it easier to strum with lighter gauge picks. The lighter picks will flex as you strum down and then strum up. The lightest gauge pick I have ever found is a nylon Jim Dunlop pick in .38 gauge. As you refine your strumming technique, you will develop the ability to flex your wrist and change the angle of attack as you alternate from down strum to up strum and you may develop a preference for the greater control of a medium gauge pick.
I find that for mostly strumming, a light to medium pick works best (for me). But for picking individual notes (especially fast) a heavier gauge works better because there is no recoil latency. I usually mix in strumming with picking out individual strings so unless it's a fast strumming only song - I usually use a medium to medium heavy pick. But I have been playing for awhile.
Besides gauge - picks come in many different materials. Many popular picks are made of a material known as celluloid. It can be flexible and is a little slippery. Hard plastic is not a material I like for picks. Softer plastic picks are not bad in my opinion. Nylon is a commonly used material for picks.
Picks also come in different shapes and sizes. The most common size is the one that I seem to always gravitate to. It's roughly the size of a US quarter but kind of a rounded triangle shape. I have tried really large and really small picks but I always go back to "standard" size.
The degree of satisfaction you get from any pick will be highly influenced by your strumming technique and the way you hold your pick. The best way that works for me to hold the pick is between thumb and first finger with the pick perpendicular (at a right angle) to my thumb with my first finger curled to point almost in the opposite direction of my thumb. I leave about 1/3 of the pick exposed.
Holding the pick a little closer to the tip will help you learn not to insert it too deeply into the strings. You just want to brush the strings with the tip of the pick so you aren't fighting against the strings so much. It takes practice.
You want to develop a smooth strumming motion that is somewhat controlled. This keeps the friction level consistent as you strum. I encourage you to anchor your elbow on the top of the guitar but avoid the temptation to use your elbow as a "pivot point" for swinging your arm.
Instead, try to relax your wrist and use your wrist to strum up and down in a smaller arc - not your entire forearm. Try to imagine using a paintbrush to paint a wall with up and down strokes. You will alternate the angle of your wrist and the brush (pick) according to the direction your wrist is moving (up or down). Also, the smaller amount of movement required by keeping your forearm anchored and strumming with mostly wrist, will give you greater control.
Also using a lighter, relaxed grip on the pick (instead of a death grip) will allow it to flex a bit between your fingers. One thing that has helped me use a more relaxed grip on my pick when strumming is using picks with some type of texture on the grip which allows me to use a lighter grip without fear of the pick slipping out of my hands. My favorite pick for this is the Dunlop MaxGrip (better than the original textured dunlop picks) made out of nylon. Below is a picture Dunlop MaxGrip Pick. It's almost impossible for one of these to slip out of your hand - no matter how loosely you hold it. It comes in different gauges.
Good luck, have fun, and keep posting questions.
In my opinion, thickness of pick does not matter all that much. Maybe it's because of the extra light strings that you're using that you do not like the sound you're getting? Imho, string thickness changes your tone more than pick thickness. The answer that Rockin Cowboy wrote was really good and exhaustive, but was maybe overcomplicating things: just try out different types of picks and see which ones you like. [But in the end, we agreed though] Nowadays, I make my own picks by cutting them out of credit cards etc. (like this) just because it is very useful and I never have to buy picks any more and I never really mind thickness.
1) maybe try using thicker strings
2) try using as much different picks as you can, just to see what you as a player like to handle the most