how do i know what battery(how much ampere and voltage) my circuit needs to work perfectly?
TL;DR: You really need to know what components you have and you need to find datasheets. In the datasheet, you find what voltage the component requires.

Don't care too much about amperes in the beginning. If you get the voltage right, each component will just draw as much current as it needs.

An Arduino Uno operates from 7 to 12 Volts, so 7.4 V is ok.

"Servo motor" is simply not enough information. There are servo motors for 4.8 V, some for 6 V and others for 12 V. There are even AC servo motors for 240 V~.

DFPlayer is for 3.2 V to 5 V, so it shouldn't be powered directly off the 7.4 V battery. Maybe it can be powered from a 5 V pin of the Arduino. Since they power it from the 5 V pin, it seems to be possible.

Also "LED" is not a term that is good enough. Is it Infrared, Red, Yellow, Green, Violet, UV or white? Depending on the color, it needs something between 1.6 and 3.0 V. Therefore, you need a resistor. What's the use of the LED? Do you need to turn it on and off by Arduino, or can it just be always on from the Lipo battery?

The L298N has a supply voltage from 2.5 V to 46 V, so 7.4 V is fine.

Next, "DC motor" is not specific enough. I have seen 3 V, 5 V, 6 V, 12 V, 24 V. 

Also, for ultrasonic, there are SR04 and SRF05. These two operate at 5 V. But if you have a different one, that may not be the case.