The fastest way to learn to map is read tutorials. If you run out of stuff Google around for some more tutorials.. Check youtube.. Etc..
If you ask a question every time you find yourself stumped it will probably take years for you to complete a map.
Here is my experience though
1. Save constantly.. Even adding a number to the end of the name to have backups of previous updates in case things go terribly wrong. (GTK will do it automatically at a time you specify but I find it easier to find the backup I want when I save them myself)
2. Don't get to attached to the first map you try to make, As you go you will probably learn better ways to do things and you will actually save time scrapping a map and starting over. Its a learning process and it takes time.
3. Open other peoples maps... if you can get hold of a .map file check it out.. Look at how they laid everything out.
A good map is not one large box but actually a series of rooms put together with sky box's to give the illusion of one large open area. That was an eye opener for me back in the day.