And when I am big, I want to play games for a living
An insei is a student with either the Nihon Kiin or the Kansai Kiin studying to become a Go professional. In Tokyo, there are about 50-60 insei. The age limit is 18 for asians and 25 for non-asians, if you have not managed to pass the pro exam by that age, you'd better find some other work.
To become an insei one has to be recommended by a pro player and be accepted by the Kiin. If one wants to become insei one should directly or indirectly contact a professional player and ask for it. As a non-asian, one have best chances with those who visit European GO Congress or US GO Congress. One should be young and talented and love GO in order to attract attention. If one is young and shows potential for the future, the actual strength is not so important.
Traditionally, an insei lives in his sensei's house, and studies with the other students in his spare time, but there are many insei living at home as well. The insei league games are played every Sunday (and one Saturday once a month). When not playing in the insei league, insei train their go playing skills by replaying professional's games, solving tsumego, and playing each other and against amateurs. There are 4 classes, A class being the strongest and D the weakest, with 12 inseis in each class from A to C and the rest of them in D. Each month, the top 4 of each class advance to the next highest level, while the lowest 4 go down one class. Once every year, the A class as well as some players from b class get to participate in the pro exam, another tournament that will determine who will be this years new go professionals.
If you want to learn more, you may want to check out the popular manga Hikaru no Go, which is set in the world of insei and professionals.