Welcome. I would suggest taking a look at the stickied messages at the top of this subforum first.
TheGrandmaster created a good rundown of the basic utilities involved
here. I would also highly suggest reading the
Influx Guide to Map Design that is also stickied there as he has some really great suggestions on how to design a map.
There's really not one way to make maps, which is a good thing. Different mapmakers have different techniques and it often is quite evident in the style of maps they make. Some people hate voxed, the editor that some with Ace of Spades. It's a clunky ten year old program, but personally, I like it... even if it does crash all the time. But my basic procedure goes like this.
Start with making the terrain with the heightmapper program. The more time you spend learning who the heightmaps work the better your map will be in the end. Then load that map into voxed and start editing. You can make KVX props (trees, tanks, bunkers, whatever) to populate your map with Slab6 or convert them from 3DS models (easily found on the internet) with Poly2Vox. Import these and place them in your map with the U key.
To keep it simple... that is basically what I do. I would suggest reading this subforum and asking questions when you get stuck, we're a pretty helpful and cooperative bunch for the most part. A lot of folks get hung up on how to use the command line programs like the heightmap utility or poly2vox.
Another good place to start might be with the
pyspades map editor. It doesn't have a lot of features... yet, but it
can be quite useful to get a mapmaker started.
Let me know if you need some more help getting started.