If you're looking to complete all of your achievements in BitLife then one of those is going to be taking up the profession of a Monk. This isn't too difficult and with a bit of know how you'll be able to complete it relatively easily! The achievement, however, is a bit more difficult because you will have to last 75 years at the job which could be problematic if you can't survive!
How to be a Monk
To become a Monk in BitLife, you will have to be a male, and then just need to get through High School. You can then find the Monk career in the job menu. I believe you need to remain a virgin, because I had trouble getting the job if I wasn't. If you can't find the Monk option available, you can either age up or close out of the app completely and re-open to get a new set of jobs.
Tips & Tricks
You can start out with pretty much any character as long as they are a male. Age up through your adolescent years and graduate from High School, but remain celibate because it appears to be apart of becoming a Monk. Once you are out of school, go to the jobs menu and look for the career. It will be at the bottom because it's a very low paying job. If you don't see it, you can age up or close out of your game and re-open it to get new job options. The job appears to be fairly rare, so you may need to do this more than a few times.
That's all you really need to do to become a Monk, but if you're looking to complete the achievement you are going to need to continue on in the career for 75 years. Make sure to keep your health up as much as possible, because that's going to determine how long you will live. You will also want to keep happiness up as much as possible and go to the doctor if you get any kind of sickness.