wondering where I should put coins into and who to level up first
~just started the game - level 5!

you should purchase 1 of each type, whichever you feel comfortable with

1 assassin
1 ranged
1 healer
1 lane pusher
1 tank

when you are just starting out, you are low on coins, so the best thing to do is level up as many characters as you can to level 5 for the 500coins

Play the free heroes for 2 weeks and decide which guys you really like from those available. After that come back here and let us know and we will tell you similar heroes that you can purchase to play.

Cheers.