is this freshwater or saltwater? if you are going to build an aquarium into a wall, a 30 gallon seems like more trouble that it is worth. wall built aquariums are usually large and custom sized... plus, if you are planning a saltwater tank, 30 gallons is a bit small. it will work but it is better to have a larger tank to provide a larger volume of water to dilute changes in water chemistry.
but if you have your heart set on it...
you're gonna need a room to build the support in. that room can store all your pumps, filters, lights, etc. your basically going to cut a hole in the wall, build a stand, and frame the opening with some trim...
add some details as to what you want, and try a google search. there are a bunch of sites that detail the project a lot better than what you are going to get in a short forum response (like this one).
DIY stand and Hood,
Coralife Lunar Aqualight (4x65W),
60 LB Hirocks Base Rock, 40 LB LR 50/50 Keys/Gulf
150 LB Playsand (~3-4 in DSB)
2 x Blood Red Fire Shrimp, 2 x Cleaner Shrimp
2 x Black and White Percula Clown, 1 x Orange Spotted Blenny, 1 x Red Stripe High Fin Goby