Thankyou for making your first post up to standards. 80% of the time, people don't on their first post. All in all it looks like an ok map for your first. I don't see a lot of advanced forging techniques used, but you'll learn that stuff later. There seems to be a lot of space inbetween each structure, where smaller structures could have been built. Again, it's your first map, so I don't really expect anything amazing.
Go here for some tips on how to make your maps more complex.
ForgeHub - Forge 101- Halo 3 Maps It teaches you how to interlock objects, put merge objects into the map, and many other techniques like how to make switches.