It took me a month or so to get used to clipless. My first shoe/pedal combo were Answer Speeder shoes and $15 Nashbar pedals. That was probably 5 years or so ago. I ran spd style pedals up until this spring, when I switched to Crank Bros Candy SL pedals. That switch took me a couple weeks to adjust to (slightly different engagement), and now I'm wearing Specialized Mtn Comp shoes.
I run the clipless on my XC FS bike and platforms on my SS/commuter/winter bike. I'd use clipless on it, too, if I had a good pair of winter riding shoes.
Practice first somewhere that you can hold onto a fixed object and practice clipping in and out. When you get that down, move on to an open field with soft grass where falling down won't hurt so much. Once you're comfortable there, move on to the trails. Start easy and work your way up from there.