playing e5

which lines are generally most useful to learn how to play against?

I really dont reccomend playing e5 simply because of the amount of opening knowledge you will have to learn. If i were you id play the scandinavisn defense which is 1.e4, d5. It cuts back on a lot of theory. But if you still want to know what lines you should look into 1... e5 ill give you a list

Italian game, ruy lopez, fried liver, scotch game, kings gambit, vienna gambit

I feel like those are the main ones you will face, there is the two knights but it transposes to the italian. I hoped this helped. And feel free to message me about any questions about chess.

With 1.e4 e5, you should spend your time on the following:

A) Find a line you like against the King's Gambit.  Simplest is to learn 2...Bc5.

B) A line against the Vienna (I suggest 2...Nf6 instead of 2...Nc6 here, far easier to understand).

C) If you remember the following in the Danish Gambit you'll be fine - White has nothing: 1.e4 e5 2.d4 exd4 3.c3 (3.Qxd4 is possible, but few play it and general principles, like attack the Queen with tempo with a developing move, is good for Black) 3...dxc3 4.Bc4 cxb2 5.Bxb2 d5! 6.Bxd5 Nf6! 7.Bxf7+ Kxf7 8.Qxd8 Bb4+ 9.Qd2 Bxd2+ 10.Nxd2 and it is equal.

D) That leaves 2.Nf3 Nc6 and now:

D1) 3.Bb5.  I recommend learning the Berlin, 3...Nf6.  Less theory than 3...a6 and it is causing headaches for White.

D2) 3.Bc4, which I suggest 3...Bc5 (less complicated) over 3...Nf6.

D3) 3.d4 - make sure you study a line against the Scotch (3...exd4 4.Nxd4 and now either 4...Nf6 or 4...Bc5.

D4) 3.Nc3 Nf6 (Four Knights Game.

Note that 2.Bc4 will usually transpose to either a Vienna or Italian Game.

Hope this helps.

Do not dismiss the Petrov 1 e4 e5 2 Nf3 Nf6. It sees much play at top level.