Before when I was weight painting, I noticed that after changing the weight of a bone some other bones would be dramatically changed as well. When I try to undo the unexpected weights the whole mesh becomes broken. Now, when I try to subtract the weight, instead of subtracting it adds. Can someone please help me? I’ve been trying for days!
Edit: I forgot to put a blend file. Here it is!
http://pasteall.org/blend/index.php?id=50825
Not unless you post a blend file for us to look at… We would just be guessing and wasting yours and our time. I could try hacking your computer, but you didn’t post your IP address either. 
Cheers, Clock. 
So sorry! I’m very new to the blender community and I forgot to put up a blend file. Here it is!
http://pasteall.org/blend/index.php?id=50825
No need to apologise! Your blend file crashes Blender every time I try to open it! Something is seriously wrong, try opening a new file and appending the objects into it one at a time and post again.
Cheers, Clock.
Hmmmm, did you make this is 2.8? it opens in 2.8, we need to know this next time!
So to the problems:
Far too few vertices to get a good deformation, add edge loops to correct this.
Skeleton not connected so deforms between bones don’t work too well. Add connecting bones, like soldiers, hips etc.
Sub-surf Mod far too high, add vertices not bump up Sub-surf mod.
Ngons - more than 4 sided polys, work in Quads - 4 sided polys all the time:
Watch Nathan Vegdahl’s Humans Riding tutorials to get you pointed in the right direction.
As a quick test I removed the Armature Mod, Parent to Armature and deleted all vertex groups - this gets back to a state where you can re-parent. then I did an Automatic Weights parent (Select mesh, SHIFT+Select Armature, CTRL+P => Automatic Weights and it worked quite well. Always try this as your first pass then tweak weights with weight painting.
Did you have many bones selected when you tried weight painting, make sure only one is. Study the tutorials and try again, let us know how you get on.
Cheers, Clock.
EDIT:
Remember 2.8 is Alpha software, expect problems and crashes…
Thank you so much this really helped!
