Download the Source SDK from the tool section. You need to own at least one Valve Source based game to use it. Once that’s done, use the Hammer editor in it. Make sure the game configuration is either set to CS:S or HL2:E2 (recommended for Gmod). Both are underneath Source 2009.
Having more Valve Source based games can help you out in terms of model and texture selection. Custom textures/models can also help you out greatly later down the road. Ignore them for now and stick with the base stuff.
Look at tutorials and the Valve Developer Wiki. Firegod has a excellent list of good mapping sites in his post.
Some various mapping tips:
-Never release your first map. It’s guaranteed to be crap. Use the first map as a place to try out basic level design and tricks. After you complete that and know your way around Hammer better, then it’s ok to start releasing any of your maps from there on out.
-NEVER EVER USE CARVE AND/OR HOLLOWING! While they may be ok in a few situations, both are very sloppy ways of getting the job done. Do things the long way and the map will thank you for it.
-Don’t use YouTube tutorials. As Firegod pointed out, certain users have a history of giving crap advice and practices. Unless they are from a respected source or are backed up by a written portion, just don’t use them.
-Start out with something small for a first map. Good places to start are things like a small park or your house. Don’t go into the hardcore stuff until you have the skill
-Don’t make gm_construct or flatgrass remakes. We have FAR too many of them and they aren’t original. Only the very skilled and talented can make a construct remake that won’t suck hard.
-Go and play lots of maps for Gmod or CS:S in a private server (to examine the level design) and online with others (to see the gameplay). Keep note of the good and bad maps, what works and what doesn’t so you can make something better.
Anyways, welcome to the community!