I've chosen an airfoil for the wing, now i'm confused weather i should do the same for tail or just attach a flat piece of wood as tail. I'm going for conventional tail design. This is my first ever plane so sorry for the dumb question.
A flat piece of wood with rounded edges will work fine.
The tail's job of producing downforce doesn't require the lift coefficients or angle of attack range required of the main wing, so the actual airfoil shape is not that important especially at low speeds, even more so under the low Reynolds numbers ("thick" air) that RC airplanes work at.
In fact most fabric light aircraft get by with what is effectively a flat surface wind deflector, like this Super Cub.
Flat plate will be adequate as most r/c planes are overpowered and do not require perfect efficiency to enjoy. Much more important are the proportions of wing, Hstab, elevator, and elevator throw. Finer points can come later. See what other planes similar to your build have.
If it is your first plane, you may be far better off buying a ready to fly. Gain some experience. There are so many variables it is best to start with an easy trainer and hook up with an instructor or somebody with experience. These can be found at local radio control flying clubs.
In the mean time, you can build simple gliders and experiment with designs. As in the early days of flight, the thin undercambered wing will give amazingly slow glides.
Also, check out R/C Groups on the web.
A flat piece of wood is technically not an airfoil due to the lack of curvature, but it may help to think of it as an airfoil with a very, very bad lift:drag ratio. The tail needs a certain amount of lift to stabilize the plane, so the more drag you incur in getting that lift, the more power you will need to overcome it. But the tail is small compared to the wing, so its overall effect on efficiency is also small.
Is it better for you to add a little more power every time your plane flies or to add a lot more effort once while building it? Only you can answer that.
Some full scale aircrafts with humans in them also have one flat airfoil as horizontal and vertical stabilizer. For instance : Laser 230 aerobatic aircraft.