There are four ways you can get into J, not counting an occasional pleasant surprise such as an operational upgrade:
1. Buy a ticket there.
2. Get a business class award.
3. Buy an economy ticket and use an upgrade award (includes VIPOWs).
4. Get an economy award and pay the difference between full Y and J.
If you're paying for your own ticket, that pretty much eliminates #1. #4 is usually prohibitively expensive too. The choice between #2 and #3 depends on how many miles you now have and how much you value getting more. I usually opt for #3.