The simplest form would be to go get some 4-column workpaper at an office supply store, get a pencil and keep track with paper and pencil.
The next simplest would be for you to get a copy of Quicken and implement it in such a way that it tracks all of your business related expenses and revenues and can provide you with a business-only P&L.
the systemic things that you'll need to do are really process oriented, not really computer oriented --> you need the ability to have a piece of paper as a source for your income and your expense information [a simple check stub could suffice for tracking your expenses].