So for stuff like armor, you can have bone attachment points. So on the arm (let's say) of the character model, you can make a point that the armor will snap onto. This is easier to rigid objects that don't deform, like armor, weapons, hats, etc. For clothing it is more complicated. Since you will have to rig the clothing with the same animation as the characters, and make sure there are no issues or else you will get clipping. In Blender (or other 3D apps) you can transfer the bone weights from one object to the next.
So after your character is animated (you can create and animate the character first, that is fine), model the cloths on the character base pose. You can copy the model vertices so they are the same, this may make some stuff easier. Then transfer the bone weights to the clothing model. Ideally everything would work, but you'll probably have to smooth out some points (especially around joints like the knees or shoulders, etc.). But that should get you like 90% there.
