There are 9 Classes. Scout, Sniper, Soldier, Demoman, Medic, HW Guy, Pyro, Spy, and Engineer.The best class for beginners is soldier, the reason is that he is a core offense class.
Scout is fast and can do good damage while close, HW Guy is strong and slow. Soldier is in the middle, He is not fast but has a rocket launcher that he can use for mobility, also has a shotgun for when he is close and has low HP or just out of ammo and can't reload.

Rocket Jumping is great for getting around high terrain, under effects that affect mobility, and fighting.
Rocket Jumping is very close to Team Fortress 2 (not exact because source has better physics than goldsrc).

To Rocket Jump look down and click with your rocket launcher, You done it! Not great but you can get better, like holding W so you fly Forward, or even hold CTRL to fly better.

Other classes are good at different tasks.
Scouts are good at flanking and getting some quick kills .
HW Guy are good at attacking sentry guns and defending objects.
Spies can disguise and backstab (like TF2).
Try to play all classes and see what your good (and bad) at.
Soldier is still one of the best classes to start with.

Spies should disguise as soldiers more than any other class.
When you are by a sentry nest (or just a Sentry Gun) disguise as a engineer.

HW Guy is very good with dealing with sentry guns.
Dispensers cannot heal (only can give ammo and armor.) so if you are from TF2 (like me) just know that.

Sniper isn't exactly like TF2, so keep that in mind. I'd add more but don't know much of it.

This is the end of the guide, I know this is kinda short.
This is also my first guide. Hope you liked it!
Thanks for reading!