sending signal to open car with microcontroller
I think the radio part is the least of your problems. To my best knowledge most car openers use a cryoptographic system called "rolling keys", which means that recording what the keyfob sent and replaying it won't work. Microchip has extensive libraries to go with thier chips intende for this purpose, but you need to sign a non-disclosure to get it. Here is a (probably simplified) explanation.