Just started playing this game (very late I know), what guns are the best?
Much more complex than that. But in general, your ideal full buy weapon would be an AK-47/SG-553 on the terrorist side, M4/AUG on the counter-terrorist side, or AWP either side. Those are the weapons you want in your hands at all times if possible. The AK is considered better then the M4s. And the SG553 and AUG are technically even better than the AK and M4 but are typically for more experienced players so I wouldn't try them until you feel very comfortable with the game overall and have a better understanding of everything in general.
Also, both of the M4's are good and it really comes down to preference however I would say most people prefer the M4A4 over the M4A1-S.
I recommend using youtube tutorials and stuff to help learn alot of what you need to know as a beginner.
Here are a few very popular youtube series that are good at teaching new players the basics.
Those are just a few, there are tons.