i'm gonna buy some speakers to my car. I got a regular radio. My doubt is: do i need to buy amplifier? Or just the speakers would be enough and right?
Radios have an build-in amplifier. You need to check if the Watts of the radio are enough for you. At the backside of the radio you can find the informations. Also you have to check the impedance. The impedance of your radio and your speakers should be the same.
If you think that your radio isn't loud enough or distorts at higher levels you can add an amplifier.
For this i would recommend an Class-D amp.
Hope this answer is helpfully.