There is no way to say what battery to use with the information given. An Uno pulls about 50mA. How much the LEDs consume depends on the resistor value(s). The motor current is also not specified. A buzzer, usually, only draws a few milliamps and is an intermittent load. The L298 is a poor choice for a motor driver, especially in a battery powered application as it is vey inefficient. Lastly, and very importantly, you have not said for how long the battery needs to last on a charge.
So once you figure out the current requirements for the individual parts, add them up (Amps) and multiply by the time (hours) the battery must last. That will be the required capacity, in Amp hours.