If the tank location is shown on the plans or instructions, go with that. If it's not, then usually the tank is located right behind the firewall, in front of the radio compartment. Having the tank on the CG is ideal, but that usually puts the tank rather far from the engine. If you have a fuel pump, then it's not a problem, but if you don't, then the rearwards tank location can lead to the engine going lean in flight, when nose-up.

Balance with the tank empty, assuming the tank is forward of the CG (as normal). The fuel will shift the CG fowards, which isn't terrible. If you balance with a full tank, and the CG shifts to the rear when empty, you may find the plane becomes unstable when it's time to land, a bad thing, obviously.

As a general observation, a Phaeton 90 isn't a very good choice for a first plane. You'd be much better off with a trainer first, to learn to fly and such. An ARF or RTF trainer will get you in the air fast while you build the bipe, or, if you want to learn building on an easier to assemble kit, there are many trainer kits that will also teach you building and setup skills that will be useful when you go back to the Phaeton.