Which opening should I use to respond white's e4? (Not e5, cuz that's way too common)

Just say any opening's name to use that is unexpected

As a beginner myself, I would advise to play the Caro-Kann because it's quite solid and fun to play. If you are careful with your pieces, White shouldn't have great tactical opportunities and that will give you time to put your pieces on their best square. I also played the scandinavian but with less success, I don't really know why

e5! anyway.

modern

Play a different opening every day of the week and keep going until you find a favourite. The fact that there are lots of openings tells you there are many respectable choices. Some will suit you better than others, but nobody here can predict which ones - it's a journey of self discovery.

Maybe, you can find something useful at https://lichess.org/forum/general-chess-discussion/i-am-probably-not-skilled-enough-for-the-sicilian-but-i-am-opposed-to-e4-e5.

The thing about e5 is it's so common that you have the choice to deviate from the most played moves in the second or third move or even later, having an efectively "uncexpected" position without having to play something excentric or that you don't like.

Philidor

I like the Caro-Kann, it's really easy to play and it's super solid. I also like the Pirc, which turns into the King's Indian.

The Czech Defense is extremely underrated.