1) Do it! It's fun!
2) A teacher will be invaluable.
3) Get the simandl method book. Read, learn.
4) A lot of folks will scoff at this...When I brought my first DB home, first thing I did was get out the tuner, and some whiteout, and put little bitty dots on the side of the fingerboard at 1, 3, 5, 7, 9, 12, etc. 4 years later, they're still there, I refer to them only intermittently, but they were of great help at first, to me, YMMV, ect. Wish I'd started at your age.
5) get a teacher

6) Whatever bass you get, make sure someone qualified puts a really good setup on it, strings, bridge, soundpost, etc. This can turn "nearly unplayable junk" into Pretty playable and acceptable."
7) Spend as much as you can, and get "a real bass," not some cheap ebay chinese junk. You'll hate it, sell it for nothing, and maybe give up before you've had a chance to play something that would give you "the rest of your life in enjoyment." Don't expect to get anything worth having for much less than $2,ooo...