First, a big-time caveat: pharmaceutical practice is a specialty onto its own, and I'm not even moderately experienced in it.
1) What is my next step to get this patented??
With all due respect to the garage inventor building a better toilet seat, pharmaceutical patents are generally not the kind you can sort of whip up yourself over a weekend. I would wager substantial sums that you're going to need a good professional practitioner with substantial experience in pharmaceutical practice. That's probably the best place to start.
You may also need some very serious research. A recent case suggested that clinical trials are required for "enablement" and therefore for valid and enforceable patent rights. The loser in that case was a university without sufficient research resources to make a valid pharmaceutical patent. Since universities tend to have more resources than many companies, let alone individuals, it seems that the requisite resources for an enforceable pharmaceutical patent are quite great. That appears to suggest that only a very few applicants could ever get pharmaceutical patents in the US since very few entities would have sufficient resources to do the requisite research.
Of course, there are others here who seem to understand that case and pharmaceutical practice generally who can say more about that.
2) How much would it cost??
A lot! (see above).
3) How would I make money on this patent given that the drug is already generic??
Your patent would not be for the drug itself but for using it to treat cancer (or perhaps specific types of cancer). A new and non-obvious use of a pre-existing (not new) product is patentable.
I hope that helps.