Which champion should I buy?

Do you know about the 3 free champions you can get? You can follow Riot on Twitter, Facebook, and Youtube to get Garen, Tristana, and Alistar free with a skin.

https://support.riotgames.com/hc/en-us/articles/202083184-Promotional-Content-Free-Swag-#h2

There are tons of options. I suggest watching professional LCS matches or high elo Twitch streams or spectating from the client. You'll find something you like by watching them get played.

don't spend RP on a champion if you can get it with IP, RP is what you'll want to use for vanity items like skins, and 400 RP is only enough to unlock a tier one champ (which you can get dirt cheap for 450 IP anyway, why waste RP on something that's already so cheap?).

Champs to look at are (get him free by sending in a support ticket about the youtube promo) (only 450 IP) (only 450 IP) (1350 IP) (1350 IP) (Get him free by following on twitter) (1350 IP) (1350 IP) (450 IP) (get her free by liking on Facebook) (1350 IP) (1350 IP).

Obviously get all the free champs ( )

that leaves mostly tanks and 2 very cheap ADCs (which you may already have)

Malphite is a lot of fun if you like playing as a tank. His ult is really strong and he becomes unkillable against a heavy ad team.

Buy a ward.

I recommend buying something that isn't an ADC. I started out ADC and that's probably my least favorite role now. But some champs to look into are Morgana, Annie, Sion, Udyr, Ryze and Soraka. All of those champs have multiple roles they can fill.

I'd recommend Jinx, Lucian and Gnar. But it really depends on what your taste is.