Caro Jann advanced var.

Dear Chess friends. Im a 1100  player. In the Caro-Kann adv var. I have 2 choises after 3. e5: 3...Lf5 or c5. Can you recommend one of these moves for my level? Best regards 

c5 is just so much better in my opinion. My rating is only 1800 so my opponents don't play the critical response (dxc5, a3), and they often play c3 which is very good for black.

c5. I would play 3...Na6, though.

I've been playing 3...c5, following the advice of GothamChess, and doing very well with it. One reason is that opponents underestimate it - they think you're playing a French Advance a tempo down, whereas really you're playing a French Advance without the disadvantage of a hemmed-in light-squared bishop.  Levy Rozman says, "You WILL win your opponent's centre pawns very often", and that's just what I did in a match game yesterday.

How do people at 1100 know so much about openings ? I for instance have never been able to understand anything about them

nerd

but seriously just understand the structures and the opening fits very well

But is it really important for me at this rating range ? Coz no matter how good I am at the opening , its not going to matter if I keep losing in the middle game 

yes I literally said understand the structures

like the advance french structure plans - open c file, invade c file, etc (black) for example.

It does not really matter. Both 3...c5 and 3...Bf5 are good moves. Just pick one and play it.