I have 2 6x9 in my car and I want another 2 more how would I go about wiring them up can I just wire them to my other 6x9 or what do I need a amp or something

the 6x9 I have in car now are 240watts and ones I'm getting are 350 watts

The wiring you need to do is printed on the casting of the amp. You will have to set some switches on the amp, but that information is also printed on the amp - do the trouble to read it.
Do not believe however that you will get more sound from 350W speakers than from 250W speakers. Or even a combination of them.
If the amp vomits 100W, then it is 100W. Whathever the speakers are.
A technical note: to hear an increase of volume, you need 3 more dB. And that is doubling the power. If you want to hear it louder than 250W, you need 500W. And, even 20W will make you deaf. Also, 2W is enough to mask police and firefighters.