A computer build needs its peripherals. At the time of purchase I already had an extra keyboard, mouse, and monitor lying around; if you know where to look keyboards and mice can be purchased at relatively cheap prices.
If you are purchasing your computer parts in one big build from Newegg.com then you may want to bundle these parts together, but you can just as easily run to your local electronics store and get a keyboard for $10 and a $10 mouse. Personally I am a little picky when it comes to a higher quality mouse, but as for a keyboard I’m using a cheap, flat, white generic one that works really well.
Purchasing a a monitor is dependent on the user. If purchasing online I would recommend stores like Amazon since they include most major brands. Monitors come in different resolutions and sizes; in terms of resolution I prefer 1920×1080 (especially if your wanting a Blu-ray drive or to download full 1080p HD movies), or if your looking to save some money go a little lower.