gozzizul,
First, let me welcome you to the forum! Glad to have you aboard!
Swanny has some good ideas, and some great pics to show how to apply a sludge wash; there are also a lot of good ideas already on this forum. I would encourage you to do a search using "wash" as a keyword. I would also recommend doing a search for "drybrushing", "pastels", and "chalks". Washes by themselves help provide depth, but additional drybrushing helps provide a weathered look that really brings the model "alive". Pastel chalks help with additional shading, such as exhaust stains, blast effect at the end of gun tubes, and rust on mufflers, tail pipes, and other bare metal exposed surfaces.
Gip Winecoff