I have been playing Tera through Steam for about a week and got a level 65 but I'm having a problem with adding funds to my account so I thought If I link a Tera account to my Steam account it will be fine but it gives a warning when linking Steam to Tera. If I link Steam to my Tera account will it reset my current hero and start again?
You have an account already, one created by Steam. It's like a normal account but not quite. Trying to link basically another account well achieve nothing - you will get an error message that it's not possible anyway. Your Steam created account won't reset (unless Support reset it manually).
What exactly is the problem with 'adding funds'? Do you mean you're having trouble buying TERA Thalers (TT)? Since you have a Steam created account, you can pay for TT (or TERA Club for that matter) only with your Steam Wallet. And how you add funds to it is another topic.