A friend of mine has asked me to do headshots for her, Ive never done those any tips?

[DavidEccleston]

The 100mm Macro should be a nice focal length, and sharp wide open, or barely stopped down to get you nicely blurred backgrounds. It will also work at a smaller aperture, like f/8, f/11, f/16 if you want background details sharp too.

Bounce your flash, preferably from above and to a side. Perhaps even behind you and to a side. Wherever you'd like the light to appear to be coming from. With a single speedlight and bouncing your light, even across a large room and raising your ISO, you can get some awesome shots.

There are many free portrait lessons available from Adorama TV, like recent "Light like a portrait master without spending a fortune". B&H's Youtube channel also has great info, like the recent direction of light talk.

edit: Oh, nearly forgot the second bit of sage advice from the direction of light talk. You need a bit of black foam, or posterboard, from a craft store, and some hair elastics. If you're bouncing the flash a bit forwards and to the side, some of the light would come straight from the flash at your subject. Block this using the foam/posterboard, held in place onto your flash using the hair elastics. This way the only light hitting your subject is from the bounce, and it also avoids blinding your model.