Very good job. For first builds, they're great. One of the most fundamental aspects of weathering is a wash. Basically, you take a darker colour than your base, and dilute it very very much. I'd suggest something akin to 1 part paint to 5-6 parts thinner (water, enamel thinner, whatever paint you use, that's what determines your thinner), then run this very thin mixture into all the recesses of the model. It'll enhance the panel lines, and really bring the model to life. Because of their small size, the shadows on a model are not nearly as visible as they would be on their real-life counterparts, so this technique will allow you to enhance all the edges of a model. From there, there are a number of different weathering techniques:
paint chipping: very basic, and very easy to do.
rusting: again, very easy to do, just drybrush some rust coloured paint onto key areas.
dustcoat: I'm going to assume you have an airbrush. A dustcoat is basically a very light misting over the entirety of the model, with a stronger focus on the bottom running gear, etc. Tamiya Buff is an excellent colour for this. Load up your airbrush with thinned down Buff paint, then just mist the paint onto your model. It is a very convincing result you'll end up with.
BTW,
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