You cant spot reduce fat mate (some people say you can but it involves controlling insulin levels etc... not something everyone can manage well enough to do properly)

Bodyfat is burned up by excersise, but will come off the body where is wants (legs, back, arms, belly etc...) no specific order or way, it depends on the individual.

To lose fat, you need to eat below maintainence calories and do plenty cardio, in a nutshell.

Doing situps will burn calories though, so no harm in doing them!