make your first financial projections
Look at your financial figures early enough and make sure you don't run out of money before you have a market ready product (don't bother with a bullet-proofed financial plan at this time).
A simply to fill in online form which calculates your startup costs up to the point you get your first revenue.
Another startup cost calculator from the WALL STREET JOURNAL which goes a little more into detail.
A very simple to fill in online form which calculates your breakeven point - the amount of units you need to sell to cover your startup costs.
An easy to use calculator to find out how long you can keep your business up and running, before you run out of money.