I was thinking about puting ghost rings on my remington model 870. But it has no rear sight and isn't drilled and taped. So what should I do?

Also are there barrels that come with the front sight on it?

you need to buy a kit and have a gun smith install it.
try brownells.  

You can also buy "Banded" front post sights.  

To add ghost ring sights, the receiver will have to be drilled and tapped.
You want to be careful who you trust for this.  
You'd think such a "bread and butter" gunsmith's job would be no problem, but several people lately have had the job botched by their gunsmiths.

The front sight is more of a problem.
First, it's MUCH better to buy the same brand front AND rear sight, since "mixing and matching" often makes problems with sighting in if a rear sight from one maker doesn't match up with the front from another.

First, for most installations the factory bead and base unit must be removed.  The base is brazed on and you either have to heat the barrel to a red heat until the brazing melts and the sight can be removed, or you can cut and file the base off.
If you go the heat way, there is some risk of damaging the barrel, and the barrel will need a refinish and the bore will need to be re-polished.

Here's the front sight options:
Buy a clamp-on front sight band unit.  This is a front sight that actually clamps to the barrel.  All you have to do is remove the standard front bead unit.

Second option, is to have a PROFESSIONAL NO-BS good gunsmith braze the front sight on like Remington does.
This will require having the barrel refinished, and re-polished inside since the red heat will destroy the finish.

The Wilson Option, is to buy the Wilson Trac-Lock sights which are some of the very best made, and are used by Remington for their Police guns.
The Wilson front is epoxied down OVER the factory front sight base after the bead is removed.
For extra strength, the front sight and original base are drilled and a cross pin is inserted to lock it all together.
Wilson will do the installation of both the front and rear for a decent price, and not botch it up.

Last option, is to just buy a new Remington barrel with the Ghost ring front sight already installed, then have the receiver drilled and tapped for the rear.

The best option, is to have Wilson do the installation.  Their sight is one of the very best, and they can do it without a lot of effort and trouble.  

I guess ghost rings arnt that important. Next shotgun will just be a factory police or maybe a wilson combat.

I bought a Remington 870P MAX which comes factory with Ghost Rings from Scattergun Tech and a host of other awesome features.  Everyone thats seen it has said it will be their next shotgun.