Don't forget to play at least one game a day to get your first win of the day bonus. Also, winning grants about double the amount of IP that losing will, so try hard in all your games! What I did when I first started playing was picked one champion, which happened to be Morgana, and only played her (she was free at the time.) From there I played her so much that I eventually had enough to buy her, and then I started filling out a rune page for her.
Getting things like Hit Point runes, Magic Resist Runes, and Cooldown Glyphs/Quints will really help out a champion, especially at lower summoner levels. I personally suggest finding a hero you really like to play, buying only them, and then filling out a rune page to make them really strong so that you can win more games and thus gain more IP. That's what I did.