This is a rabbit hole of personal opinion and creeping requirements. Here's mine and some ideas:
Anticipate where you might go before just getting speakers. Just replacing speakers with more expensive ones won't do much unless there are other improvements as well.
If you're not up to it, find a friendly car stereo installer - shop around till you find someone you trust.
Figure out the radio first - what is the ohm rating on the outputs? What is the wattage output? RMS (root mean square = average) and peak? Stereo out (4 wires)? Front and rear(8 wires)?
Almost any 6" round (or 6x9) with a second dome tweeter will be an improvement (assuming you've matched the ohms to the radio). Some come with 3 speakers in a 6x9. About $100 / pair and up... They will be sized for a car door-sized space (meaning 3" deep or more) so they probably won't fit where your original speakers went.
But now that you have better speakers in new housings, you'll likely find your wiring is limiting, or your radio isn't that good - meaning it can't adequately move those speakers.
You might as well pull new wires while it's open. Depends on length, but putting in 14ga wire will probably make a big difference. Wire choice is woo woo on the interwebtube forums. Go to HD and get duplex 'speaker wire' - it's fine.
A small amplifier will make a huge difference with those new speakers even at lower volumes. Here's one, but there are lots....
And with a new amp, you'll want to revisit the wire choices to make sure you have enough ampacity to get the sound to the speakers.
And you'll end up wanting some more bottom end. You don't need to rattle the windows, but it's just better. ;>)
