find the square root of the complex number 1 + 2i?

Find the square root of the complex number $1 + 2i$

I was trying this question many times, but I could not get it and I don't know where to start.

Actually after squaring the $1+ 2i$ I got $-1 + 2i$, and I also tried multiplying by $1+i$. However I don't know the exact answer.

Thanks in advance