I'm going to very briefly cover the basics of rigging in this guide. Rigging is very simple once you get your head around it, and it's crucial in more complex mods.
1. You're going to want to select the Skin modifier. This is going to turn your skin darker or to blank mesh while it's selected, depending on what version of 3dsmax you're using.
2. Check the "vertices" box.
3. Click what bone you are having problems with or want to improve. I'm having problems with the arm sticking to the sideof the model, so I'm going to click "R(ight) forearm".
Now, what do all these colors mean? Red (assigning the vertex 1.0) means the mesh/face/whatever you want to call it moves on a 1:1 ratio with that bone. Similarly, if you were to assign some vertices .5, it would move at a 1:2 ratio, meaning half, and the mesh turns this orangesh color. Of course, then there are hierarchies and all this other crap that you don't need to worry about right now. This guide is more for just fixing little bugs you run into, rather than rigging an entire new skeleton and shit from scratch.
4. Select the vertices that are being dragged with the forearm.
5. Now, scroll down a little in the side window until you see "abs effect" under "weights properties", and then enter 0 into the box.
It's partially fixed. What's still wrong though? You see that the spine is still attached to a bone it's not supposed to. Let's speed this up by going straight to the spine, or spine1 in this case (spine1 is the upper spide/chest area, and spine is the lower abdominal area).
6. Click spine1 in the side menu and select the same vertices as you did before. Where spine1 and spine meet, you're going to want to assign the vertices .5 in spine1 and spine, rather than just 1.0 in one of them. This results in smooth animation flow.
Usually where one bone meets another (for example, r forearm to r upperarm), it's better to assign the border of vertices .5 in each bone.
That's pretty much all there is to it. Go around, making sure the nothing's assigned to what it's not supposed to be assigned to. If you click on r forearm and it's blue around the midsection, select the vertices in the midsection and assign them to 0. It's really that simple. It takes some practice and playing with to get used to, but once you do, it makes modding a hell of a lot easier. Good luck.
If you have any tips, suggestions, or questions, feel free to post them and I'll respond to/answer them as soon as I can.
special thanks to Popsicle for teaching me his ways and to Zenophobia for helping me retrieve this
Note: gonna get back up to speed with 3dsmax and include some more examples like clipping