www.sheldonbrown.com and www.parktool.com are 2 excellent sites with maintenance tips and instructions
Depends on what the trouble is. www.sheldonbrown.com, www.bicycletutor.com or www.parktool.com can tell you all about it.
try this web site http://techdocs.shimano.com/media/techdocs/content/cycle/SI/Tourney/RearDerailleur/SI-5V80A-En_v1_m56577569830612967.pdf
Shimano has both internally geared hubs and externally geared hubs, and the method can be a bit different depending on that. For external gears/assuming you're not cross-chaining - simply lift it...
Too little info to answer here. Check out the posted link for repair tips.