Woah dude. Hold your horses. Dont throw money at the game just yet.
There are 10 free champs EVERY WEEK. They rotate thru all the champs over time. (yeah I didnt know this at first!!)
You should be playing EVERY SINGLE ONE OF THEM every week. For each one, make a CUSTOM GAME, with 4 bots on your team, 5 on theirs. Play the new champ.
This way you have an easy game usually and can leisurely try out a champ. Do it ONCE for each one.
This lets you determine who you like and dont like. Even if Dont Like, you know how they work! This is important later.
When you find one you like, Buy them AT THE END of the week... in case you change your mind. No reason to buy them while they are free.
The champ packs arent all that great either anymore. Same price but many of gthe champs are old, outdated, and also cheaper to buy now anyways! (i.e. 6300IP champs reduced in price to 3100,etc)
If you get bored with the 10, buy the 450 IP champs. Theres 6 of them and that gives you a nice roster-bump to work with.