I don't know identities but I've learned sin^2(@)+cos^2(@)=1. Try doing the tagent with sines and cosines.
tan(@) = 1/2 = sin(@)/cos(@)
Square both sides, and multiply acorss to get cos^2(@) = 4sin^2(@). Then add a sine to both sides to get cos^2(@)+sin^2(@)=5sin^2(@)=1. Then sin^2(@)=1/5. Do the square roots to get the answer.
I don't know what identities you have? The way the other post gave might be as good as any. But if you have to use something else, you could do 1 + tan^{2}(theta) = sec^{2}(theta) = 1/cos^{2}(theta) = 1/{1 - sin^{2}(theta)}, and plug in that 1 + tan^{2}(theta) = 5/4. The answer works out the same.
draw a triangle of tan(theta) = 1/2 use SohCahToa (Sin-opposite/hypotenuse)(Cos-adjacent/hypotenuse)(Tan-opposite/adjacent) once the triangle is drawn, use the pathagorean theorem (a² + b² = c²) to find the hypotenuse. after all sides of the triangle are found, use SohCahToa to find sin(theta).
First you should draw a right triangle and label the hypotenuse X and then the opposite side 1 and the adjacent side 2. That is what is known. Then by doing the Pythagorean theorem 1 squared plus 2 squared equals X squared. That will give you the X value. Then Sin theta will equal 1 over X.