My car speakers for 2006 Ford Fusion?

Okay so I have a pretty sweet set up; I have Pioneer subs, a pioneer stereo, and I'm getting speakers. My other pioneer stuff works great so I think I'm going to get pioneer speakers too. What sizes will I need? I'm pretty sure I'll need 4 5x8s and 2 6x9s. But what are tweeters? Do they go into the 6x9 spots? I'm pretty sure I only need one is what I've heard so do I buy one 6x9 speaker and then one tweeted? Help me! I know a lot about cars, just not audio stuff

Most standard speakers are made up of a woofer and a tweeter (2-way). Many have more than one tweeter (3, 4-way -- these are not necessarily "better"). For quality sound there's no need to add add'l separate tweeters.
To make a big improvement in the speakers' performance you may want to add a 4 channel amplifier. I doesn't take much power to make a big improvement over your head unit's power. Something in the 50x4 range would make a big difference.

Yea you will want a separate amp to actually hear a difference with the 6x9s. A head unit alone doesn't send much power to each speaker. You won't notice much of a difference between those and stock speakers without an amp. And the speakers you actually hear music coming out of have tweeters. Tweeters are what play the actual singers voice in a song. Along with some of the highest frequencies you hear. You won't need extra tweeters since you already have speakers that have tweeters. It wouldnt hurt adding extras but you won't need them. The only benefit of buying tweeters, which are generally 1.5-2 inches in diameter by themselves, such as with component speakers, would be to put them up on the dash or front pillars to point them at your face. Which would make the highs sound slightly louder