I'm a beginner and im thinking of getting thicker gauge strings and a guitar store employee told me that if the thicker string doesn't fit around the nut I'll have to go and get my guitar a setup. If I get strings with thicker gauge that does fit without getting a setup is there any potential damage to my guitar?
Swapping to the next-thicker (or next-thinner) gauge of strings of the same type (from electric to electric, or from acoustic to acoustic) is normal and almost never requires any sort of setup adjustments.
For example, my guitar had 9s on it when I bought it. I have used 13s on this guitar without needing to modify the nut.
However, if you want to put acoustic guitar strings on an electric guitar, the gauges can be much larger and warrant the use of a luthier's narrow files to enlarge the grooves in the nut to accommodate the thicker strings.
You can do a quick check before you buy the strings:
- remove the new string from its packaging (ask the manager first)
- gently lift the old string from the nut and hold it aside
- see if the new string touches the bottom of the groove in the nut
If it touches the bottom, no modification is necessary.
As you get more experienced, you may still benefit from a professional setup to match the string action and intonation with your playing style, string gauge of choice, tuning, etc. But for now you probably don't need a luthier's help.
Thicker strings engage more force. I've heard of a case where the guitarist changed strings to thicker ones, and the neck couldn't support the force needed and hence doubled.
I suggest you take your guitar to a luthier who can inspect it and advise you on your specific case (guitar/string gauge).
Hope this helps!
You will definitely need a set up with a gauge change, there is no way around that.
But if the thicker string doesn't even fit in the nut groove it will not be properly mounted and likely slip off the nut and that will cause damage to the nut and the finish on the side of the fingerboard, not to mention the guitar will likely not be playable. In the rare chance that the string stays in place it will be too high to fret the first position, etc.
By the way, fixing the nut because the string does not fit properly is more work than a standard set up (I think). Changes in weather, any playing, will also cause the set up to slip out of adjustment slowly over time. It has been recommended to me that I get them set up at least once a year, maybe twice. So being a guitar owner comes with that maintenance responsibility.