It depends on what kind of music you want to play. I would suggest starting with guitar or piano - basically the instrument thay will be responsible for melody and overall shape of your music. Vocals also fall into this category, but I think that for most people it takes a lot more hours to be a good guitar / piano player than to be a good vocalist.

After some time, try to include drums. It's a completely different beast and also takes time to practice.

Bass guitar you can leave for last, because when you can play guitar and drums it is more or less a formality. Not all techniques, of course - to be good at slapping takes a looot of practice hours.