I am making the assumption that you are borrowing the board and binding.
Since you are a slim girl and given you need a wider board; a softer flexing mens park board like the EVO maybe easier for you to learn on. For comparison, a (women's version of the SL) NS Infinity has a flex of 3, a NS SL has a flex of 5, the NS Proto has a flex of 5 and a NS Evo has a flex of 4.
I tried to find a link for the Evo or Proto on a weight vs board length to give you an idea of what lengths (range of board sizes) would help you. But the best I could do was the following.
Backcountry.com Sizing Chart
Also, I would follow the advice about using wired's explanation on boot sizing. For comparison, I wear a size 8.5 Celsius and ride a 157 Proto and weight 170lbs. I'm not thin, so I have to size up to get the extra edge for the ice (Ice Coast).
Hope this helps - Nito