I make a solution of sodium carbonate(0.05N) .What is alkalinity of sodium carbonate(0.05N)?why?

What do you mean by 0.05N? Or do you mean 0.05 M?

If the latter was the case, then, you need to have the dissociation constant of sodium carbonate to find the pH of the solution (or pOH)

But Na2CO3 is definitely alkaline. If you don't know the method to calculate pH, you might want to try adding some indicator. Methyl orange is yellow, phenolphtalein would be purple, litmus would be blue in alkaline medium.