First goal: keep your carry alive (this may not be your adc).
Let your creativity shine through your abilites; Renata is a difficult champion due to her long cooldowns and ingenious skills. If you want to master all of her strength, try crazy things! Don't forget vision control is important as well, make sure to buy control ward as much as needed!

Second goal: put tremendous pressure onto the enemy botlane.
The combination of your E + auto/passive is deadly! Make sure to use this combo while controlling the bushes to nibble down enemies health bars. If the two botlaners are close by, land a Q to throw one into the other (this is easier if you slow with your E first). In addition, shoving is extremely easy with your E, but keep an eye on the enemy jungler!

Third goal; get everything out of your passive.
Your passive is a strong tool that you should never forget about! Generally, attack a minion after poking an enemy to reset your passive on them. Try to force objectives like dragons and heralds (they lots of max HP!). Make sure to target the same enemy as your carry so that your passive deals extra damage. You may also make farming under turret easier for your adc, as killing with the extra passive damage still gives them the gold.

- Always aim to poke with your E + auto (this is harder against pyke).

- Deny dashes by spamming your Q toward the threat.

- Generally, use your Z on a mid-life ally (this may be yourself!).

- Being healed, shieldied or put in stasis increases the duration of your Z's zombie state.

- Do not use your ultimate if the enemy cannot target any ally nearby.

- If you fail your Q by grabbing a minion, throw it onto an enemy champion to deal slight damage.

- Use your E to control the wave.

- Your Z is primarely an offensive tool, use it to provide movement speed and attack speed to your carry (not always the adc!).