There are such RFiD reader IC's, that you connect via SPI to your uc; most of the work is done in this one ic and you need some passives and an antenna. For example, NXP has PN7xxx and CLRC663 series; ST has some and some others as well. The manufacturers provide software libraries to talk to most common tags without too much work. You can copypaste the evaluation board pcb layout for easier hw development. If you are not developing your own board, some vendors (ie sparkfun) sell arduino-like modules etc.
When designing your own board, the antenna tuning is a tricky thing to get right (there are some capacitors, inductors and resistors for matching the on-board antenna to the chip). With simple tools and some trial and error, you can get decent tuning (ie 3/4 read range of the best possible); for very good tuning, it takes a lot of specialized tools and know-how. Also, the enclosure and all nearby metals to the antenna affect the tuning, so going with an existing reader may even force you to re-tune the antenna matching circuit it read range is important for you.