Expeditions are your main means of collecting resources (So yes, you get fuel, steel, ammo, and Bauxite this way).(rjd also has good advice)
What you need to do when you when going on an expedition is to remember the following:
Did you remember to fully resupply your fleet? If so, you should be fine.
Is your fleet or flagship at the apporpiate level? Some expiditions require flagships to be specific levels in order to successfully beat them. If you meet the requirements, then you should be fine.
Do you have the right fleet composition? Some expiditions require certain ships to be in your fleet. Most of the early ones require at least destroyers and light cruisers to make use of the expiditions.
My best advice I can provide you regarding expeditions is to look things up here
http://kancolle.wikia.com/wiki/Expedition