Title’s pretty self explanatory.
One of the things I did was look at some robot reveals from past games and the current game. You can learn about a lot of different mechanisms and design from past robots.
As for building, it really takes practice. A great source of information is Youtube. There is tons of tutorials on there. Past threads on vex forums is also another good source. Building robots will quickly get you learning how to have good build quality.
Heres an example of a past thread that I learned a lot from.
Hope this helps!
Not to mention, the forum itself is a great way to learn about VEX. This is my first year in VEX and the forum has helped me greatly. Pretty much every robotics topic I can think of has been covered on the forum at some point.