It depends on what you want. Some people like to set up 3 speeds to use the middle gear on flat terrain, the low gear for climbing and the high gear for downhills or strong tailwinds.

Other people find 3 speeds more useful with the top gear used for level ground leaving the lower gears for climbing and accelerating.

There isn't any particular reason you need to change the crankset on your bike. You can either leave the extra ring(s) on or get chainring bolts designed for using a single ring on a double. If you have a triple, just remove the smallest ring.

You may wish to use a different size chainring but how many teeth depends on your terrain and whether you want your most used gear to be 2nd or 3rd. I would check Sheldon Brown's gear inch calculator for some help deciding.