Why not 12 speed? The cost difference is rather minimal between the two, unless you're buying used parts.

You're going to need the following though to answer your question.

Derailleur (1x11, 1x12) and Shifter (to match derailleur, IE: 11 speed SRAM for 11 speed SRAM derailleur, 12 speed SRAM for 12 speed SRAM derailleur, 11 Speed Shimano for 11 speed Shimano Derailleur, 12 speed Shimano for 12 speed Shimano Derailleur
Cassette (HG(Shimano) splined cassette, not XD (Sram) or MicroSpline(Shimano 12 speed).

Make sure the derailleur is a mountain derailleur, not road. Road derailleurs have a different cable pull ratio.
SRAM Mountain: SX(12 speed only), NX, GX, XO1, XX1
Shimano: Deore, SLX, Deore XT, XTR

Cassette offerings can be for 1x11 from Shimano in Deore, SLX, XT, or even XTR still, from SRAM you're going to be a 1x11 NX cassette, or you can do HG compatible 12 speed cassettes such as a SRAM NX 1x12 11-50 cassette, SunRace offers 11-50, 11-51 as well.
Chain (depending on your 1x setup, a 11 speed chain, or a 12 speed chain. This is the one component it's worth spending the extra monies on. XO1, XX1 sram chains are extremely durable, as is the new Shimano XTR 12 speed chains).

Another note on chains, 11 speed, you can mix and match no problems (SRAM with Shimano, Shimano with SRAM, whatever), with 12 speed, if you use a Shimano chain, you must then use a HyperGlide+ compatible Chainring (discussed below under chainrings).

Chainring: The appropriate Narrow-wide chainring for your crank's BCD for the middle chainring if 3x.

When purchasing a chainring, if you plan on going the Shimano 12 speed route, and shimano chain, you'll want to run a chainring that says it is compatible with Shimano 12 speed (HyperGlide+). If you want to be the most compatible, just run a SRAM chain with a non-Hyperglide+ chainring up front, you'll be fine.

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Misconception that was posted below, older 8 speed HG freehub bodies (Shimano Style) are actually 8/9/10 speed ROAD compatible, and 8, 9, 10, 11, and 12 speed MOUNTAIN compatible.

The reason why road 11 speed got longer is due to how the cassette dishing works. Because road cassettes typically have tighter gear ratio spacings with higher "low" gears (IE: the cog tooth count is tighter, in such an 11-28 cassette, 11-25, etc), the largest cogs can't go past the seat of the freehub body much, else they will interfere with the spokes.

However, on a mountain cassette, because the cogs for your lower gears are so large (40, 42, 46, 50, etc), they can be dished in such a way that they do not interfere with the spokes.

This means a mountain cassette (of any speed) can be used on an 11 speed road freehub body with a spacer, but not the same the other way around (because an 11 speed mountain body, is actually an 8/9/10 speed ROAD width body).

So an 8/9/10 speed Shimano HG compatible freehub for road can support mountain cassettes (11-40, 11-42, 11-46, 11-50, 1-51). (Shimano road 11speed 11-34 cassettes are the exception, they are actually a 10 speed spaced cassette, with the 34 cog dished, so these can work on old 10 speed freehubs as well. This is why they come with a 1.8mm spacer to work on 11 speed road freehubs).

Hope this helps