A couple questions...

1. Where do I put my downloaded maps from this site?

2. How do I make maps of my own?

1) You need someone to host it for you, or you can host it for free on
a free server like geocities (other people will tell you better ones).
Then provide us with a download link to download your map.

edit: sorry I mis read your first question, im a moron.

2) you need Hammer, if you own the HL2 game with steam, then in the
Play Games window go to tools and right click on Source SDK and hit
install, once it is installed entirely (total 220mb) then run it, and
click on Hammer. This is what is used to create the levels, search the
internet for mapping for Half life in general (sine the dynamics of
mapping for both games are almost the same) to find tutorials for the
most basic map (there is tooo much to do to list here)

C:Program FilesValveSteamSteamAppstwokniveshalf-life 2 deathmatchhl2mpmaps

That's the path for HL2DM maps. Just put them in there and they should
show up in the in-game list. Substitute my username for yours obviously.

LOL. Question 2 helped.

You only slighlty misread number 1. You see, I downloaded a map FROM this site. What folder do I put it in so I can use it in the game?

here: C:Program Files\Valve\Steam\SteamApps\YOUR E-MAIL\half-life 2\
deathmatch\hl2mp\maps\ folder like TK said, only there were no ""
marks. if you downloaded CSS maps put them here:

C:\Program Files\Valve\Steam\SteamApps\YOUR E-MAIL\counter-strike source\cstrike\maps