I am 100kg and started 8 months ago with a 8'11" (actually, started with a 7'6" Sealion but didnt go anywhere).
You can definitely start with a 9' board, *IF* you plan to use it only in glassy conditions (If there 15knts chop I fall as soon as I stop paddling), and if the board is wide enough, and has a stable shape (not all shapers master how to make boards both lively and stable). I went paddling on the flat or micro waves for one month each morning before attempting waves. My board is the fabulous Drops Vulcan,
www.gong-galaxy.com/magazine/pics/1-hour-4-boards/Try to find a good construction, solid and light, beware of the prestigious brands where you actually buy a low-quality china pop-up heavy and floppy with a famous sticker on it.
Note that I ordered now a bigger board (9'11") to use with chop and to go to our "outer reefs". Do not go too small fi you have any chop where you surf.
My advice would be to get a big (wide especially) board, but one with enough performance that you will want to keep it for small waves (or choppy conditions) once you you buy a shorter "shortboard feel" SUP. Do not buy a heavy tank.