Hey there!

For applying for SAT, you first need to register yourself on the offcial site of College Board, the link to which is: https://collegereadiness.collegeboard.org/sat . Also, you don't need to convert your marks into GPA. Most colleges would want your original transcripts/ results, however, if they need it to be converted to GPA, they will mail you the instructions to do so or you can email them regarding this. You can expect a response within 24 hours from their side. Hope this helps. All the best!