you can look up all sorts of guides, as people have already said. but mainly, the biggest point to any class is optimization.

As a warlock, you just want to figure out the best combination of stats and spells that yields you the most sustained damage.

Are you attacking 1 target? then you probably want to use your best single target attacks. make sure you keep all your DOTs (damage over time) up. and cast all your big-hitting spells in between. this will help you do the most DPS (sustained damage per second)

Are you attacking 3+ targets? then you want to be doing your AOE (area of effect) spells. Putting DOTs on multiple targets and then using reign of fire will usually yield you the most damage in large mob scenarios.

Also, get use to using your big CDs (cool downs - spells that have wait timers after casted) in harder scenarios. If you're leveling and you accidentally pull extra targets, blow some big CDs to help you with the increased trouble. Are you about to die? Get use to using your big defensive CDs to try and keep yourself alive.

Being good at the game really only means that at any given moment, you know what spell will be best to use to combat that moment. So just get familiar with your spells. Take notice of which ones do a lot of damage and which ones dont, and why or why not. Notice the stacking effect of your spells. Does Immolate cause another spells damage to increase when its up? Which curse will do the most damage or increase my survivability? Which pet is best to use and why?

As you go through this learning process, you'll easily find yourself neglecting some spells, using a lot of the same spells, or ignoring some spells entirely.

The other thing you'll learn is how to read your abilities correctly. Sometimes you'll get a new ability or have an option for a new talent spell. Its easy to get lost in the whole "wow this looks so cool, i would love this ability". But you have to look at both the benefit and the practical use. Sure this ability is cool and may be nice in rare situations, but how often are you really going to use it? Is it better to have a 10% increase to Immolate (which you use on every single target) or 80% increase to a spell you rarely use? the 80% increase may seem more flashy and awesome because "holy shit 80%?!", but in actuality, the 10% increase will actually be way more beneficial because you use immolate all the time, so you get way more use out of that increase.

I know this is much more general than 'press this and then this and then win'. But when i first started playing in 06, a friend laid it out like this for me, and it really helped with understanding how to play well, and what other players do to play at a high level.

Learn your class. Learn your spells. Understanding why one spell is better than another is everything.