Tips from someone with a bit of experience:
1st tip: Plan ahead! I have several sketches of possible tf2 layouts on my bedroom, so I work on then. To get a good layout, see what VALVe has done. Some are ingenious (Nucleus), some are interesting (Badlands), some are just straight forward (Granary), some are simple, yet with something that marks the map (Gorge). Change the layout, create new doors, close old ones, change cp location, until it's your map now.
2nd tip: Related to my post in your other thread, it was just something really stupid I did. But I learned the basics. Everything you do gives you another bit of experience. Or do you think Coldfront came out that awesome just by luck?
3rd tip: Go at pubs, at lobbies, whatever, collect feedback based on yourself and other players. In CTF, people want Scouts to get the intel. In 5CP, people attack at one point and leave a defense at another. What are the most exciting periods? Why?
4th tip: Read tutorials, explore the dev wiki, learn design and detail theory, check MangyCargace's Frontier Essay, if you didn't got all of that, learn. Learn. That is the key part. You can combine all of those into one map, but it still will have its flaws. How you can fix? That's the art of level desing. Predicting the changes you will have to do. If you do something wrong, change. If you do something right, keep it. It's just like that. No wierd ass thing.
Basically, learn what you can do and how you can do something on Hammer.
Oh, and the "block" from the Block Tool is called a brush.