I'am new in the field of rocketing and space,i wanted to know what do we do by adding different stages to a rocket and it's used for what, and if we add a second stage we can put another booster on it ?
To get a rocket to space, you have to build it to be really light while containing lots of fuel. This is called the mass fraction: In most rockets, more than 90% of the starting weight is fuel, the rest is divided into the rocket structure (engines, tanks) and the payload (the satellite you want to launch).
If you build one really big rocket, it starts out with a good mass fraction, but pretty soon those tanks are mostly empty and the engines push along lots of weight that's no longer needed. It's pretty difficult to build a rocket that has a good enough mass fraction to get to orbit with a decent payload.
But there's a shortcut: if you can throw away the empty tanks, your mass fraction gets a lot better. This is what stages are all about: instead of building one really big rocket, you build 3 smaller ones and stack them on top of each other. Each part (stage) has engines and fuel tanks, and is thrown away when its tanks are empty.
When you do this, you can also optimize the engines: the second stage has much smaller (=lighter) engines than the first stage, because they're pushing less weight. And you can modify the engines to work better in the environment they're in (the first stage has to work at sea level, the second stage can be optimized for high altitude or vacuum operation, this changes e.g. the size of the engine nozzle).
This is the Saturn V:
You see a large stage at the bottom, and smaller stages on top. After 2 minutes and 40 seconds, the first stage is empty and is discarded.
The major limitation to the performance of chemical rockets is the amount of fuel they must carry to reach high speeds. To reach higher and higher speeds you need to carry exponentially more fuel, because you have to accelerate all the fuel you need as well as the payload.
By splitting a rocket up into stages, you can discard some of the weight of fuel tankage and unnecessarily large engines progressively during the flight, making the rocket lighter and more efficient as it goes.
More information can be found on Wikipedia.