no hot water in shower but fine everywhere else
	
	
		hi.
so I know NOTHING about plumbing.  The water that comes from my shower is lukewarm at best.  All the sinks and the other shower are fine so I figured it's not the hot water heater.  I've looked online a bit and discovered it might be "debris" in the valve.  However, I don't know exactly how to fix this.  I've seen suggestions of:  turn off valve (which I can do - I think), disassemble value and check for debris.  I'm lost at the disassemble part.  How do I do that?  Water really won't come gushing out?  Should I turn off all the water in the whole house?  
is this even the right solution. 
I noticed this to be a problem last winter but seemed to go away until recently again - or did I just not notice it because it.  I'm a new homeowner with little cash and ambition to learn so can you help me do this myself or do I have to call a plumber?