OK, first install the
ABS mapping pack linked above. Also grab
Acegimko's Enviroment Gallery &
Ravidge's Lighting Library while you're at it.
Next get out a pad and paper and sketch your basic layout. This is crucial in my opinion, good planning before you start can be the difference between scrapping a map and making an enjoyable one. Once you have an idea of how your layout will work block out the map and check the scale is ok by doing a fast compile and having a run around.
Next add your gameplay and lighting entities. These can be taken from Ravidge's Lighting library and ABS's gametype library included in the Mapper's Resource Pack. Next get some environment entities in their, you can use the one's in Acegimko's Environment Gallery to get you started, they're basically the environment entities from the Valve stock maps.
Once you have all of these things and the map compiles you have an alpha release. Get it out there and get testing it to make sure it all works as it should. You'll have to make several alpha iterations until the gameplay is right. You can also start texturing and adding props to areas of the map which you are sure are working well. Once you're quite certain there will be no more major layout chages you can move onto beta release, which basically means that any further large changes will be mostly cosmetic.
Once your map is at a stage where you're pretty much done release a release candidate. There should only be minor fixes to make at this stage.
A word on optimisation, start early. If you get your areaportals and such in ealy it will save you time compiling and it will also make life easier down the line as you won't have to sort all those things out once your map is full of props and detail.
Most importantly, have fun.
Check out this thread too:
http://forums.tf2maps.net/showthread.php?t=8595
Good luck.
