If you want the short answer to this question then here it is; it is unlikely you will ever be a professional bicycle rider. Honestly; it is one of the most difficult things in the world to do. There are very few professional competitions out there, and to take part in them you need to be incredibly good. Wait; you need to be absolutely amazing. If you want the long answer to being a professional bicycle rider then you should read on.
Firstly; there is no such thing as a generic professional rider. You will have to have a specialism. Do you want to professionally ride trails? Want to road race? Sprint Trials? BMX tricks? You have to know what you want to do. You are also going to need to get very good at it too. It will take many years of practice, and many hours of practice by the end of it too. You may not even be successful at the end of it all. Hopefully you end up having tons of fun along the way though!
If you wish to be a professional rider then you are going to need to enter as many amateur competitions as you possibly can. You need to win them too. Quite often sponsors will head to amateur competitions in order to ‘scout out’ people who have potential talent. Who knows; if you really outshine everybody else in that event then you may just get an offer to hit the big time.
I have seen a number of cyclists get professional sponsorship by entering amateur competitions and by now it seems to be the only real route into the professional level of the sport. Of course; winning some amateur events may give you a ‘pass’ to a professional event but nowadays these seem to be few and far between.
If you end up getting sponsorship money, and I must again stress that this is quite rare in itself, then your sponsor will be funding your training. They will also try and get you into professional events. The majority of sponsors out there will know the cycling industry pretty well and thus this should be fairly easy. Your only job is to train and train.
Do however remember that the sponsor is most likely not going to be giving you that money out of the goodness of their heart. They are giving you that money because they want to see a return on their investment. If you do not generate them a return on their investment (i.e. start to win events) then they will ditch you pretty quickly. This is why it is important to work hard.
If you enter the world of professional cycling then you will be up against some of the best. Nobody expects you to win the event obviously, but they do expect you to give people a run for their money. If you are consistently near the lower end of the results table then your professional bicycle career will be over pretty sharpish.