I'm still taking my first steps in to making a 'proper' map, myself. I've never put out one of my own new maps before. I was put off for a long time because I didn't want to make something bad. But I realised that everyone's first work is bad, you just don't see it. It doesn't mean you are a bad mapper if you make something bad, it just means you need more experience or need to change something. If you carry on working on something, taking your feedback seriously and changing things over time, eventually you will have a good map.
My biggest piece of advice to anyone making their first map is not to overscale it. I tend to make my maps 1.5 times the scale they should be, and I end up in tight spots where I have long sightlines and boring open areas. Some people like to place spawn points in Hammer to simulate the size of a player, and I think that does help sometimes to remind yourself not to go over the top.
I used to want to block everything out in 128 but that can lead to overscaling very easily. 1x 128 is good for narrow walkways. 2x128 is good for a compact corridor. 3x128 is a nice average size. If you go beyond that you might find you are mapping for giants and have trouble filling things in with detail.
Try to come up with a theme for your map in your head before you set out building it. Get some playable areas properly planned. Imagine what kind of interesting architectural things your theme could naturally create. When you build it, keep it simple, don't spend time on detailing unless it's necessary to highlight an objective or provide context, and try not to disallow certain kinds of play unless you feel it could be overwhelming. For example, building places for engineers is fun but if they get too much cover, their sentries could cause a real problem in the game. Keep sniper sightlines at 2000 units max in open areas, 2500 units max where there is lots of cover. Sentry range is 1100 units radius so try to plan around that sometimes. Provide two or three routes to a place as a rule of thumb. Make sure you have plenty of light during your alpha stages and don't compile in HDR until beta.
Test your spawn rooms and resupply lockers before you submit your map for public testing.
