I am building a high wing nitro plane in coroplast. Just stuck while building wing part that is, Does a high wing plane needs a dihedral angle? 
How about flat spar?

it depends on the model type like a trainer, sport or aerobatic etc and do you want the model to self-correct or level itself ?

YES, unless you have functional ailerons.

You can verify for yourself by building a small balsa or depron winged chuck glider without dihedral!

My wing airfoil is Clark Y airfoil and flat spar. So does it affect the performance.

And I do have Ailerons and Flaps too

Let me clarify the purpose of dihedral first.
Assume there are no ailerons, and the wing is flat.
If the plane banks for any reason, it will sideslip to that side (say left side)
The airflow now is slightly from the left, instead of from straight ahead.
So it strikes the fin/rudder from the left, pushing it towards the right.
That makes the nose turn left.
So now you have a plane with left wing down and nose turning left.
This will ultimately lead to a spiral dive into the ground, unless corrected.

How to correct?
If you have functional ailerons, you give right aileron to level the wings.
If you don't have aileron control, you need dihedral.
What does the dihedral do?
As the model slips to the left, the left wing presents more area to the sideways airflow than the right wing.
The left wing therefore generates more lift than the right wing.
This opposes the bank and levels the wing.

Edit:
Just saw your post saying you have ailerons.
In that case you don't need dihedral.
Unless you need some built in stability in the roll axis

If you have already flown one/two trainers with dihedral, you can move on to a non-dihedral wing with ailerons. If you are just starting to fly, dihedral is required.

The airfoil has nothing to do with dihedral (ignoring washout, different airfoils at root and tip etc, which are beyond the scope of the present discussion)

Further, when you say 'flat spar', i presume you mean a wing without dihedral.
In the accepted terminology, a 'flat' spar means a spar placed in such a way that its width (along the wing chord) is more than its height.
Usually the spar is placed in such a way that its height is more, ie, 'upright'

A flat wing of course means one without any camber/airfoil.

So i guess what you have is 'non-dihedral' wing, with a Clark Y section, and an 'upright' spar!

BTW, posting details/drawings of the model, and building/flying experience makes it easier to give a useful reply (instead of the theory lectures that i often end up giving  )

Next question: Why did you opt for flaps?
I recently got a 36" span Yak 12 that has working flaps that are totally unnecessary!

 I am not a beginner and flying even 3D planes. My doubt is totally cleared. I know it's my stupid one. so, I am just continue with non dihedral and Clark Y section wing. I have attached my half finished plane.

Unusual for a 3D flyer to use a Clark Y!

My similar model has a symmetrical section and full span ailerons/flaps.

So why are you building a trainer?

I have planned to build for shows to perform drop some stuffs, toe a banner and many.

Pl AVOID!

Too risky for onlookers.

Too risky for you if anything goes wrong.