1) Think about the music you enjoy listening to the most and what instruments are most prominent in it. That's the instrument you should choose. Like getting good at anything else, practicing music is hard and can be chore at times. You won't always have the inspiration to play. The best way to consistently practice and improve is to be learning song that you really want to be able to play.
2) Playing with others is absolutely essential. You will improve more jamming with others for an hour than you will in ten hours of practice on your own. You do need some amount of skill to do this, but not an awful lot. Once you have a little bit of confidence built up, look for others at a similar skill level to meet once a week or so and play together. You don't need to form a band or anything, but you just might find that you want to.
3) Music theory is nice to know, but not essential at first. It is very useful when writing music of your own and improvising with others, but there's a bunch to nail down first before not knowing it will hurt you at all. Learn basic notes and chords, then focus on picking songs you want to know and learn to play them well. While learning songs, think about what music theory you have learned and how it works within the song.
4) The truth is that becoming "good" will take a decade of dedicated work. But if it is something you truly enjoy then after a year or so you will be able to get to the point where it is both fun and very rewarding. So really do focus on your enjoyment of it. You don't have a parent requiring you to practice for an hour a day anymore, and you won't find immediate benefit from it at the start. Anything you can do to make it an enjoyable routine is very very important. I won't recommend specific books or methods because you fall into the same problem as telling someone how to learn programming. There are so many different options, but what's important is to find what you like the best.