BA's quick guide to weight painting

At the bottom of the 3D view where you switch between edit and object modes, you can select weight paint
after selecting it go to the right hand side bar

Above the sidebar where the import/export buttons are, their is a wide row of several buttons.
One with a tiny sphere and cylinder is probably highlighted

Click the one with three dots connected in a down pointing triangle

You'll be in the right place, if there is a menu with with several entries that have bone_0, bone_1, bone_2, etc...
You should be able to click on each entry and it will show what vertex are assigned to each bone, because they will turn red

So to assign the vertexes to say, the left shoulder, you'll have to click on which ever bone number makes
the left shoulder and upper arm glow red

With the cursor over the 3D view hit "t" key to open up a side bar on the left

It'll probably have some kind of picture at the top and under it, there will be a slider for "weight" and "strength",
click and drag both to "1.000" if they aren't set already
under it "Auto Normalize" should be checked, if it's not click to turn it on
and blend should be set to "mix"

You should now be able to "paint" more red onto the parts you want to move with the the bone you've selected
to make things easier to see, you can hit the "z" key to turn off the textures and make things bright

If you are careful, on the 3D view where you switch between "edit mode", "object mode" and "weight paint mode"
there should be another button, called "limit selection to visible"
it will have a square with dots at the corners and a faded version of it slightly behind it

Clicking this button will allow you to paint the front and back of the part as well as the hard to reach bits
simply by click and dragging over the part
you can toggle it on and off to make painting easier, just be careful since you can paint through the object
and paint stuff behind it

Anyway, if you want to switch which bone you paint to just select a different one on the right side bar
and paint the model you want attached to that red
then export and test if it works