I want to upgrade my car audio,but, what to do first? HELP!?

I have a 2003 Honda Accord Ex 4 door. I was really hoping to drop a double din stereo ,but, crutchfield doesnt carry one that fits. I saw one guy on youtube install a DD radio in his. Some ppl say get a good radio and it'll power your factory speakers. Then I have other ppl suggest i start with replacing the factory speakers, 6 1/2 front doors and also 6 1/2 behind the back seat along with a 4 chnll amp. I was looking at component speakers b/c of better sound quality but not sure if i need components in the back or do i. Im really new to all this car audio stuff but i know someone on here has the expertise that might steer me in the right direction. Can some one recommend brands or online stores to buy from. Thanks in advance for your input.