By viewing at your profile, it looks like you already played the upright, is that correct? The first and foremost thing that you should do is find a Double Bass teacher and start taking lessons A.S.A.P. He/or She can recommend what pieces that you should undertake at the appropriate time.
I began playing bass in middle school and high school, well before I'd ever had my first formal lesson when I was a Junior in high school, that was a big mistake. I was able to compensate for it over the years, but it took a lot of catching up. Do yourself a favor and try to avoid this.
Ric Vice
