I cannot see the detail on your model very well, but provided you have some recessed lines left on it, I would recommend that you coat the entire model with some gloss coat (a lot of people like Future, which is acrylic and will not damage your paints no matter what type they are). After the gloss coat, I would apply the decals, then another gloss coat. After that, you should be able to start your weathering.
There are plenty of articles and how-tos on this site if you do a search, but I would work on the panel lines first, since your model could use some work in this area. You want to do a wash of a very thinned out black/brown mixture of paint (acrylic would probably work best since it doesn't usually damage the paint) and apply it with a brush going through each panel line. After 15-20 minutes or so, you will want to remove the excess with some moistened q-tips (water for acrylic paints).
Check out
this website for videos showing how to do most of what I just wrote and a lot of other things.