There are certain basic "rules" for being a pro musician. By the time you've aquired the necessary ingredients, you wont have to ask this question, you will know.
1; A very good command of sight-reading, and I dont mean that TAB b.s. Also a knowledge of the Nashville system would'nt hurt.
2; A quality "vintage" style bass, such as a Fender P, a quality "modern" 5 string, a quality fretless, and possibly an electric or acoustic upright, also an acoustic bass guitar. This would be minimum for a pro. Many, like Will Lee or Dave Pomeroy will have 10 or more basses to call on.
3; A quality studio rack containing tuner, compressor, and other bits and bobs, plus a live stage rig able to handle any situation.
4; The ability to compose or improvise bass parts instantaneously.
By the time you've aquired all of the above, you'll be on the way. Then it's a matter of networking, making yourself known to all and sundry, chasing every lead you can, maybe moving to a large city where the opportunities are greater, gaining experience playing at jams and with bands, playing ALL styles,
OR.........
you could luck in with a band that becomes succesful, even though you can only play on two strings and dont know the names of the notes on your bass. Dont laugh, it's happened! Just dont wait for it to happen, you've got more chance of being struck by lightning.