please use the program "paint" next time to save your model screenshots in the jpeg format, most people with slower connections wont want to download it and not give any comments.
IrfanView is another free program that will make the screenshot making business a lot easier.
Its actually not bad for your first model, the proportions feel ok.
To texture this model, each component of the gun needs to be uv mapped, each component is considered a limb, each limb has a shape, that shape has to be virtually flattened out so you can paint onto it.
each limbs 3d shape needs to be flattened out on a virtual piece of paper using a process called uvmapping or uvwrapping.
From there you can use a paint tool to apply the texture.
Imagine a cereal box with the stuff taken out and the box flattened out by pulling it apart from the glued areas. same principle with each model and models limb.
there are many uvmapping tools, a popular one is lithium unwrapper.