This looks pretty good for your first map!
A few things I noticed:
-This map has a distinct lack of flank. Add some more routes around the point, or else the map becomes tight and chokey
-I agree with what Kraken said, but I'll elaborate: Your map really only uses 2 basic heights, and even then they don't extend through out the whole map, so open up the space a bit and add some cliffs, some ledges, and some higher grounds!
-Last is a suicide arena. Make the RED spawn a bit of a ways away from last, so they have to walk, but don't make it on the WAY to the point. Otherwise BLU must resort to spawn camping to win.
-Does RED really need a forward spawn?
-This map doesn't need 2 checkpoints. Put one in the middle in between the 2 you already have.
-Move BLU spawn to be around a corner, and give it multiple exits. That will probably fix the sightlines, but add a bit of cover just to be safe.
-Those trusses are a good idea. A VERY good idea. They add a proper dimension to the map and will make it more fun to play on.
-If you want to make 2fort bridges like that, either make them actual bridges with water underneath them, (which will actually HELP with the height variation) Or make them proper buildings.
And one more thing: Don't give up!
There have been thousands of mappers that quit at the a1 stage. I almost did myself, but something convinced me to do otherwise. (probably my maps being comedically bad

)
This is great for a first map, and there have in fact been examples of small payload maps working out in the past. Heck, one guy was in almost the exact same situation as you:
http://forums.tf2maps.net/showthread.php?t=21833
Keep mapping, and good luck!