The most important thing to do with any blunder is to stay calm. Evaluate the position objectively: pretend you've just been handed a chess board and told to finish the game.

In this position, you've got a well-defended e-pawn and a weakened kingside, especially `e3`, `f2`, and `g3`. Your King Knight's usual best spot is unavailable. However, the position is certainly not lost: there are many perfectly good opening lines that feature an early `f3` from White. Taking some inspiration from them, you I might be inclined to go for a setup with `Ne2`, `d4 (exd4 Nxd4)`, and `Nc3` to establish a strong center and dominance over the light squares. 

I can't overstate enough, though, that the most important thing is to just evaluate the position objectively and try to set your ego aside when it comes to past moves, especially mistakes.