*continued* It's simplest to build one that is one to two feet tall. The simplest one to build is a SGTC. The easiest way to build one is with a transformer, connected in parallel to a spark gap connected in parallel to a capacitor connected in parallel to the primary coil and a secondary coil of about 1200 turns of magnet wire
placed inside the primary coil and a topload (it has to be of a shape with no edges, such as a sphere or a toroid--donut shaped) on the secondary. The secondary is then grounded to a copper pipe in the ground. The transformer needs to be at least 7500V 30mA. Yeah, DO NOT TRY IT AT HOME unless you know what you are doing or want to electrocute yourself. SGTCs cannot be audio modulated. It can be with an extra part but the noise would be drowned out by the spark gap. If you yell and can still hear yourself, you didn't make the spark gap right. To make sure you get the values right, you can go to JavaTC. It's a free online software for SGTC building. You can download it online. For more details on how to build one, buy or torrent
"The Ultimate Construction Guide to Tesla Coils" or something.A slightly harder one that requires previous knowledge on electronics is a SSTC. They aren't as hard as DRSSTCs, but these ones are nice because the arcs are pretty safe to touch without a faraday suit
and they are the fun, audio modulated ones.
I like them tesla coils huge.