Before anything I assume you have proper safety gear? Full leathers/good helmet/gloves/boots/backprotector? If not you need it. start shopping at someplace like www.kneedraggers.comSully said:
As for what bike to the track?? Take whatever bike you wouldn't mind bending into a pretzel and blowing up. If you can't afford to destroy it...don't race it.
Like everyone has said its best to get lots of tracktime before jumping into racing, the pace some of the fast "Amateur" guys run is quite fast in 600 classes. You can get started doing trackdays with ( www.nesba.com or www.sportbiketracktime.com ) and get up to speed and get used to staying smooth with other riders REALLY nearby at speed. When you feel comfortable do the www.learningcurves.com school and get racing.
Racing is expensive.....very. Typical racing weekend can be an EASY $500+ by the time you include tire cost/gas/race entry/gate fee....ect Like I mentioned previously sponsorship is for the REALLY fast or the successful business owner.
You can look into contingency money, which is the lowlevel version of sponsorship... basically you run company X's product and have thier stickers on your bike(and fill out the required forms) and if you place well you can get $$ or free stuff from them. Other than that you can do the CCS purse races (Am Unlimited GP) which pays about $200 for a win and down to $65 for 10th place...?
Bottom line is unless your superfast, don't count on sponsorship money. If you find you ARE superfast...you need to get exposure on a national level. At that point you'd need to start looking a F-USA classes or AMA racing. :evil