At lower levels make use of the fact that players aren't good yet in your matches and try out a variety of Champions using the free rotation if you can (I regret never playing top Lane at lower levels because now I'm hopeless there).
Do NOT buy any runes until level 20 when you can get Tier 3 (I know you didn't ask but it's Good advice in general). Aim to save around 10k ip for when you hit 20 so you can load up on decent runes at 20 (only a suggestion, they're not needed until 30 so splurge on more champions if you want).
For champions, I liked to start cheap (450ip or 1350ip) unless I really liked someone I played for free. Good examples are Annie, Ashe, Warwick, kayle, jax, master yi, dr. Mundo, Morgana, and Evelynn.
Note you can get 3 champions, tristana (adc) , alistar (support) and garen (top) for free WITH A FREE SKIN by liking/following/subscribing to league of legends on Facebook, Twitter, and YouTube if you so choose (you can search how on league of legends home page).
Hope this helps!