total newb to my xterra audio... looking to undo my mistakes

So I have a 2003 xterra totally stock

I'm looking to upgrade sound/ kill road noise at the sametime.
I never even knew I had rear speakers even.. lol

Either way I replaced the head unit with a generic bestbuy cheeapo to be able to use Blue tooth.
Replaced door speakers when they died with generic speakers too.
Tweeter fell out one day I just glued it back in.
So its been neglected to sat they least.

But now want to "upgrade" the interior sound instead of just noise with a rhythym

I'm confusd by a lot of terms, is there a good sticky with info?

1-Should I replace my HU and with what for good medium level quality.
Same goes for front and rear speakers.

I still use the rear for cargo etc So I dont think I want to add any spekers to the rear that stick out.?

I can upgrade speker wire too just dont to what form what I curretnly have
I am a total newb when it come sto audio. so I'm looking for direction/ help

I'm not looking for a high end system I'm all for a good high end quality but on a budget that does better than stock.