Renata is a very classic support character that still bends toward an agressive kind of play. Her abilities usually pair quite well with ADC's that tend to go all in and less towards long range marksman like Caitlin (although, I know, everything can work with everything).

Q - Renata's Q is more than a simple imparing move. She can root and throw her opponent afterwards in the direction she wants. Very useful to allow your adc to get to the opponent but it can also be a way for you to get out of a bad situation - which is very useful considering Renata's lack of movement. I recommend to use it with special care when an enemy with a dash is chasing you. Try to use it after the dash is used, or while he is using it.
You can combine this (offensively or defensively) with your E. (More after)

W - Renata's W seems like an interesting boost but in my opinion, isnt a dealbreaker. Its main use to me is to allow your ally not to die. Enemy champions will often stop to attack the ally you empowered in a teamfight because they know that it means a rez. Dont forget that you can use it on yourself and that its a beautiful way to catch up with this annoying enemy that is JUST out of reach.

E - This is your bread and butter, mostly if you want to include a Staff Of Flowing Water to your build. Your E goes as far as it can go even if there are minions or enemy champions on the way and it affects every ally on the way, INCLUDING yourself. It will shield your ally but also damage and slow the enemy on the receiving end. You can use it if you manage to catch someone with your Q to get the enemy closer, you can then shield yourself and slow your target and let your teamates (or yourself) finish the kill.

R- Your ultimate MUST be used when there are more than one opponent in front of you. For (initialy) 1.5 sec, they will turn on each other even though your team keeps firing at them. This will come in handy in ganking situations or at major objectives like dragon or baron.

You have to be carefull early considering your lack of damage and resistance. Also, make sure that you dont steal some minions away from your adc with the splash damage of your E, which is a very common mistake when people play characters like Lux. You have a good amount of mana which will allow you to stay in lane late, but unless you have a very good synergy on your side or some very good hooks with your Q, wait at least until level 3 to go full engage.

The moment you get your R, you can stop worrying about mass ganks in your lane (well, dont fully stop worrying, but if you play well you can avoid bad situations easily)
Renata is not much of a roamer but she can be extremely usefull to a jungler at dragon since she can boost his attack speed and shield him. Make sure to keep your Q for a random opposing JG that would come say hi and steal your objective.
When you see an opportunity, you can counter-gank IF you have ult up. If you do, make sure to get close first, use E then use ult.

Renata can shine late game but her damage can become quite irrelevant. NEVER roam late game, stay with a bunch of allys and make sure you shield them good and boost them up. Renata's R usually comes in handy in late game team fights, if you score a good ult, it could mean a win for your win.