Are there any nice websites that offer this?

maybe youre a user that can teach me.. idk

do you have any background in theory? instruments like this require a little bit of musical knowledge to play well. i picked up my buddy's bass and played it a little. it's the same as guitar (EADG), just picked a little differently. look up tabs to your favorite songs and play along imo

I would suggest you to get books and read about the basics.
Checking online video can also be helpful, but the best would be to get a teacher, because you will need that extra ''motivation'' if you really want to learn.

Websites that offer ''good'' lessons are usually not free.
If you know how to read tabs and only want to play some songs you like for fun, then it's another story