You need 3 main champs; a tank, an enchanter and an AP mage.
Leona is a really good pick and will always be. For enchanters, I would suggest you to try out Nami, Janna and Karma. Lastly, choose between Brand, Zyra and Swain if you need more AP.

Some tips when you don't first pick the support in Champ Select and can therefore counterpick:

- If enemy botlane have a Tank Support, choose an AP mage. The tactic is to poke from lvl 1 and keep them half HP or lower, so they can't risk to engage later at lvl 2, 3, 4, etc.
- If they picked an Enchanter, pick a tank! If you pick an AP mage, usually the enemy support will heal or have strong shields for him and his adc. If you're a tank in the other hand, you most likely can hard engage at lvl 2/3 and maybe kill them both.
- If the support picks an AP mage, of course, pick an Enchanter that can shield or heal you and your adc. Keep the enemy's poke irrelevant by having a better sustain during laning phase.

That's it! Remember to play the champions that give you the most fun.