Very difficult to give answers with so little information. Choose the most sensitive 2 or 3-axis accelerometer from the cheap end of the price end. Make sure that the sensor has an interface that you can use (analog, i2c, spi).
Try to find a module that is already soldered to a pcb because most of the accelerometers are in very small "no-leads" packages and are therefore difficult to solder (for hobbyist).
i want to build two wheel self balancing robot,
with geared dc motor, at a size, which can be easily balanced,
later i want to mount other parts to the robot with the self balanced base as the basement for the robot.
I used one of these for a robotics project recently and it proved very sensitive and easy to use. If you leave the g select pin disconnected it will be in 1 g mode, besides that just give it power and pull the slp (sleep mode) pin high to turn it on.
Hey...
I can advise you ADXL series , i tried that before in some projects related to heading and balancing UAVs. you can check for more details according to your needs.