What should I do to become an SDM? I have completed my B.Tech civil engineering with 70%.

To become a SDM you should qualify state PSC in which you can get this post.

Here are two ways to become as a SDM. The no. One is UPSC-Civil IAS Services and State Public Service commission. Out of these two UPSC-Civil services is the best one. So appear in above two competitive exam clear all the formalities as prescribed by the exam conducting authorities.