Total newbie here, but screw it, I'll try and think of some tips anyways. Take these with a grain of salt.
1) Your job when mapping is to create an interesting and fun experience for players. Try and imagine the different ways that they will flow through areas and combat each other. Make combat interesting (with height variation, cover, etc), don't just put your players on a flat empty plane.
2) Don't overscale or underscale your map. Walk through it with an average-speed class like Soldier to get a sense of its scale.
3) Playtest often. Don't do any detailing/art before your first playtest, or else it will be hard to change the layout. I made this mistake.
4) Put layout/gameplay before aesthetics. You should never give up a good layout just because of your map aesthetics.
5) Don't except your first map to be great. 99% of people, including myself, start out with shit-maps. There's nothing wrong with shit-maps as long as you learn from them.
6) Read the tutorials and things on this website. Personally I like grazr's in particular, especially this one:
https://tf2maps.net/threads/guide-scale-and-your-map.12605/
7) Don't use any big gimmicks for your first few maps. Things like water maps, vertical maps, etc. It's hard to pull off gimmicks, and it's much easier to just stick to the normal TF2 theme.
8) Look at Valve maps and see how they do things. Look at their layout, the ways that players flow into areas, the ways that they can fight each other. Look at their aesthetics and details.
9) Don't lose hope. If you receive negative feedback, don't worry, you'll learn from it. In mapping you learn from your mistakes very quickly. I'm still a newbie, but I already feel like I've learned a lot!
10) Constantly shitpost on the steamchat