No need to get registration with ST. You are entitled use the benefit of SSI. Or you can make use of the Notification 33/2012.
There are two option
1) Pay tax and take input credit, if your total turnover doen't cross 10 laksh, even though you need to pay tax. and take credit of inputs which is used in or in relation to provide output service.
2) Once you cross the SSI limit, then you can get register with ST department start paying tax.