Ok. I just searched around and thought I should elaborate.

If you're looking for gears while maintaining the appearance of a fixed gear, you're most likely looking for a coaster hub and a shift mechanism that either takes place internally in the hub or is "discretely" changed with some kind of lever on the handlebars.

You want a coaster hub because that will keep you from having to put on either disc or rim brake calipers, ruining your image and street credit. You could just ignore that and Ted-Shred around but that's really pushing this fixie aesthetic to the absurd.

You're going to need some kind of internal shifting mechanism. Really, you should just make up your mind and ride the bike that makes the most sense, be it a fixed gear or bike that conventionally shifts, but if you're adamant about this, internal shifting will kinda look like a single speed. I suggested a kickback hub since that will keep your frame options open and save you the aesthetic-defeating necessity of running cable.

Wear a helmet.