there are few ways to join RAW:
1) you can join through armed forces army, airfore, navy n all the other special forces in it.
2) Through civil services like IPS, IFS n so on.being an IPS officer u can join in CBI, RAW n can remain in state police services.Its your choice.
3) If u r a civilian u need a high level recommendation, even through u get in u probably find desk jobs. Its better to enter RAW through IPS...so check out UPSC related website for the exams n requirements.you must put a great hard work to pass these exams n interview.
All the best