Don't buy RP yet. Only use BE, which you earn by playing games. You wanna be practicing before splurging $ on champs you won't know how to use.
I'd actually recommend a few:
Top Laners: Nasus, Malphite, and Darius.
Mid: Zoe, Syndra, LUX!!!!
Jg: Udyr, Yi, Lee Sin.
Bot ADC: Ez, Caitlyn and Trist.
Bot Supp: Nami, Leona and Sona.