Depends on what you want to do with it and how much you want to spend. You really need a bi-directional tool unless you just need to pull codes. Launch makes the x431 ,it's 800 bucks well worth the money. Stay away from AE and read the reviews my best suggestion.
torque pro app is also good for reading codes, and monitoring operating conditions.
cost is $4.95. then you need some type of obd adapter. they range from around $25 to $100
i actually have two adaptors, this obd link mx bluetooth adaptor i use with the tablet: