Scouzer, I've been looking into making my own map as well, but I can't seem to get myself started on it. I have no ideas!! However, you say you're looking to do Combine/Rebel battles. I don't know what exactly that is, but it sounds like you may want to alter models and weapons in addition to making maps. If so, you're looking to actually make a mod, which is decisively more difficult than just making a simple map.
If you are wanting to make a simple map, here are a couple of links I found in my searching:
Hammer tutorial. This was made by a user in the Steam forums, as you'll see it's simply a post. It could be explained in clearer terms, but it works for the most part. I was able to start with this and quickly get a room going that I could run around in, took maybe 15 minutes.
List of cvar's. I've read that if you make your own maps, these console commands might come in handy to test stuff once you know what you're doing. These may or may not be helpful.
VERC.
(This site is sometimes slow, so if it doesn't appear the first time, keep trying.) This apparently was one of the best resources for map making with early CS. It seems as if a lot of the tutorials with the old SDK is still valid with the new one, but don't quote me on this. Some searching around this site should prove it handy tool that you don't want to be without, complete with forums.
Hope these help.