Slr Tip For Outdoor Scenes Outdoor Portrait Practice?

Outdoor Portrait Practice? - slr tip for outdoor scenes

I'm new in photography and try to get some practice in my friend and agreed to 2 of its friends, let me practice on them. This is a portrait session outside. My question is: I have the Canon EOS Rebel XS with Lens:
Canon EF-S 18-55mm f/3.5-5.6 IS Lens Reflex
Sigma 55-200mm f/4-5.6 DC Zoom Lens
Canon EF 50mm f/1.8 II Camera Lens

What goal should be used as a priority. I know you want to use the 50 mm in some, but not sure whether the shoulder to use as the main objective, and if so other lenses that are best suited for these vaccines?

So, some good advice or links to useful information about outdoor portraits. Thanks