That's a simple question, the answer would require books to be
exhaustive. Just a few ideas:
White should not be too nervous about getting the pawn back. So is
Qa4+ a way, but after Nc6 or Nd7, black has easy play.
Black should not try too much to hold on to the pawn. He will get
behind in development.
A few lines, played very frequently:
1. d4, d5
2. c4, cxd
A. 3.Nf3 and now the most common lines are
A1. 3. ...., e6 with lots of different moves for white & black; in a few
moves, white will (after e3...) take Bxc4 back
A2. 3. ....,Nf6 idem as above, sometimes variations cross each other
A3. 3. ..., c5; 4. d5 or 4. e3
A4. 3. ....a6, not with the intention of playing to safe the pawn with b5
(altough it is playable if white continues with 4. e4, b5; 5. a4, Bb7
because e4 is attacked. very complicated though. my last game like
that was a loss against Bessyboo) but to play after 4. e3 the black
moves e6, Nf6, C5 ,b5 , Bb7(it is called fast fianchetto) and Nbd7.
B. 3. e4 makes it an open game
B1. ...,Nc6; 4. Nf3 or Be3
B2 .... Nf6; 4. e5, Nd5
B3. .... e5, 4. Nf3, exd or Bb4+
B4. .... c5; 4. d5 or Nf3
Of course there are lots more, but these are the most common.
sin.