I say you shouldn't hurry in buying a new champion. Provided you are level 30 already and have a minimum experience of say... 1000 matches in total, you should start picking your favorite position. Then focus on that position more, but learn others too, because it'll help you in soloque. Then buy champions according to that position. If you already have some, learn those first, buy the necessary runes, buy at least 5-7 rune pages and only then start getting new champions.
Buying new champions without mastering at least 2-3 is pointless. You'll never improve.