It can be a good opening if the rest of their opening moves are good. It's advantage is that it quickly develops a piece and can control the center somewhat. However, it fails to put any pawns in the middle or open lines to develop the bishops. Against this, I would put pawns in the middle squares, develop pieces, and castle.
Move knight on first move?

Its a perfectly fine opening. The right word is flexible.
I play 1...c5 against this hoping to transpose into a sicillian.
yeah, 1 Nf3 is a good opening. 1. Nc3, not really though. Here is a GM game from the Queens Gambit Declined. ANAND VS KRAMNIK 1996

If you mean the White player choosing 1.Nf3, it's a good move. Theoretically not quite as favorable or practically as popular as 1.e4 or 1.d4 (honestly I would just choose one of those two).
White can maintain an opening advantage with best play but it allows so many possiblities, it could transpose into almost any Open Game line or Closed Game line, or something different altogether. You are basically asking Black what he would like to play instead of choosing the direction of the game yourself.
1.Nc3 is not very strong because often you want to advance your c-pawn, and the Knight would be blocking it, and it does not allow the King to Caslte Kingside (which is usually prefered) any more quickly.
1.Nf3 is honestly rather flexible, and a move I'd play due to the number of transpositions it may move into. It would probably mainly be used for a king's Indian, as that is a supposed "weapon for all". e4 would also be a move you'd have to look out for. I suppose it'd probably just transpose into a '1.e4, e5 2.Nf3'.
A lot of my friends like to open the game by moving there knights forward. Is this a good opening? If so what is its advantages? If not how can you counter it effectively?