Not bad at all for a first map. With some work this would make a great arena style map for multiplayer. I won't rate it yet, but you can make some easy improvements:
First of all, the keeps are too close to the edges. If this is a multiplayer only map, it would be okay, but you'd have to let us know in the map description. Give those AI's room to build.
Second, I can tell you wanted to make a balanced symmetrical map (equal land and resources for all four corners of the map). However, the balance is not quite right. The team keeps on the left are much closer together to each other than the team keeps on the right. Also, the river down the middle is off-center. A great way to make sure you're getting perfect balanced arena maps is using a grid map. Pat DarkKnight made some excellent ones, which can be found here:
Third, the map is blocky and a bit sloppy. Please be more careful and use more detail. The landscape tools on the map builder gives both large and small brushes to build your landscape. Making a map is like painting. Use the big brushes first, and gradually go smaller to put in details.
Don't let this discourage you. This is your best map so far, and with a little work it'll be a good one. If you like map making, keep practicing, and you'll get great at it.