Introduction to the atomic chess opening "Two Knights Attack" (1. Nf3 f6 2. Nc3).

after 2 ... c6 - Mistake. I recommend 2... Nh6 instead. However c6 is played frequently - especially amongst lower-rated players, so it makes a lot of sense to learn the lines.
e6 is the most common defence after 3. e3.
3...d5 is also common. With correct play from both sides, white wins a pawn. 4. Ng5 Here black's best option would be Bg4. fxg5 also gets played, but that's worse.
Exchanging the queens is another option, but Qf5 makes sure that you keep the initiative.

After 2... Nh6, you have several options. One of them (although not objectively the strongest), is 3. Ne5. (DO NOT do 3. Ng5 because that's a win for black)

3. g4 is objectively the strongest move here. It's also the sharpest and trickiest one for both sides.
Here are dozens of lines for that lovely opening...
Later I'm shall upload some studies related to that