I've been trying to rig this character I made, but his sleeves keep clipping into his arms. I tried using the weight paint with no luck, although I'm new to blender so I'm probably just not doing it right. If anyone could take a look and help me, it'd be greatly appreciated!
You need to bring some corrections in Weight Paint mode, because as we can see, some bones have influence on this part of your mesh while they should not, so activate a brush with the Blend mode > Substract and the 2D Falloff option enabled.
Also, in the Armature modifier, enable the Preserve Volume option.
Perhaps you should also reconsider 2 things: You have a lot of faces, maybe you've applied a Subsurf modifier, was it necessary? I think it would simplify your work to keep a small amount of faces. Also, your armature rest pose should be a T pose, it would help when you parent the mesh to the armature, but to do that you need to modify the armature in Edit mode, then reparent the mesh.
Unfortunally there's a bit less Information in your question and - AFAIS - not any object in your uploaded file? So a real general answer...
If you are starting with parenting, maybe you're looking for Parenting with armatures? Creating Bones for Rigging in Blender
Maybe the problem is cutting in cloths in ellbow and popliteal? Then IMHO the better solution is using Cloth physics. A workaround will be using additional bones for sensitive parts.
You can learn much about using weight paint on different bones, if you import a Make Human character (I'm using collada to import) and study the dependicies...
I hope it helps.