I began mapping and need help
 

I started mapping a while ago , at first i made some small things like a room with a stair (small parts of lvls) but i want to make real ones . I started making a new omaha beach far different from the original and you can go behind it in the forests . good for sniping and stuff.i also made a part of a real life german destroyer . i can make it all but i deont know how to texture my lvls . can anyone please help me? also how do you place objects like a tank or a sandbag?