Seems that you are new to the whole synthesis thing and you are looking for specific sounds found in other songs, so I recommend you to start with a software synthesizer that has a big and good library and macro support/dynamics.
The library will let you choose from an array of well-organized pre-programmed sounds, and the macros will let you tweak those sounds providing a simple interface with just a few controls.
I see that you want "dirty dubstep" and "electro" sounds. Well, Native Instrument's Massive is a famous synth in dubstep and electro production, it has a very well organized library, and provides a very simple macro interface.
In fact, that's a true for every single synth Native Instruments produces (that is not hosted in Reaktor). All stand-alone NI synths have the same library and macro system, so you can't go wrong with them.
Let's see what I mean using Massive.
For a newcomer that screen can be very intimidating. Not everyone is a sound designer, and not everyone wants or needs to be one. With Native Instruments synths you don't have to deal with this screen if you don't want to, and still tweak the sounds easily.
In the upper right there are three buttons, go to the Browser section.
Here you will be able to select a patch based on its characteristics. Here I told Massive to show me patches with the characteristics: Instrument - Bass, Source - Analog, Timbre - Hard, Articulation - Percussive, Genre - Techno/Electro.
On the right a list with patches (sounds) that match your criteria is shown. Just click on them to see how they sound like. You don't have to select all those characteristics, you can select only one, or none, or whatever.
See those 8 knobs at the top? Those are the macros. That's all you have to tweak. You can completely forget about the intimidating first screen. This is very comfortable and productive for non sound designers.
Absynth and FM8 provide similar library and macro functionality.
As for DAWs, Ableton Live provides a similar library and macro interface, using the Instruments Rack library, in the Instruments directory.
Here from the browser we selected the instrument rack Synth Brass Silk Horns. Again, the patch is reduced to the 8 knobs that are presented to us in the lower left part of the screen. Pretty convenient.
That combo of Native Instruments synths and Ableton Live will get you pretty much any popular sound you can think of, and if you eventually are interested in sound design and diving for new sounds becomes your thing, that same combo is amazing too; because of both the synthesizers and the synthesis environments (MAX for Live, Reaktor).
As for production techniques, start simple. Replace one instrument with a synth. As you get more practice you can mix in both an instrument and a synth that are around the same frequency range.
You will notice that synths tend to need a lot of equalization (sometimes the patch already provides some). Bass synths tend to need some attenuation in the lowest freqs, modulation-based synths (FM, AM, Phase Mod, etc) tend to be very bright and need some attenuation in their highest freqs, and so on. Shelf EQs are great on these scenarios. Be careful when using filters like LPF or HPF, those might kill the tone too much or get rid of the fundamental.
