Put your models on a surface with good lighting (if you don't have any lamps, use a window sill with some nice sunlight). Also try putting your camera on the surface to reduce camera shake and if it has one, use the macro button (shaped like a flower) for taking pictures of small objects.
However, if you are looking to weather stuff, it is a good idea to start small and subtle - from the looks of your rhino (from what I can tell) you have kind of just coated a lot of the tank in grey paint. Your best bet is to work on small scratches first, working up to large areas of dirt and dust etc once you get confident in where weathering will be occuring, and have a better idea of the effects you want to achieve.
As for weathering tutorials, check out this site: http://tutofig.com/?s=weathering
- compiles vast numbers of tutorials onto one site.
But for basic scratches, dents and chips, paint a small area of your metal colour (for example gunmetal), then touch up the top edge with a slightly darker version of your base colour (so it looks like it is shadowed), then do the bottom edge in a slightly lighter version so it looks like it is catching the light.
You can go further and use another colour layer to represent the "primer" sprayed on before top coat is added using the same method (ie if your tanks are blue, you might have a kaki primer underneath, so use a dark kaki on the top of the chip, and a lighter version on the bottom of the chip).