By default, you are a cash-basis taxpayer. You record and pay taxes on income when it is actually paid, regardless of when the contract is signed or the invoice is issued. Your first quarter estimated taxes due April 15 should reflect actual income paid to you between January 1 and March 31. Don't count as income, jobs that have been invoiced but not paid yet.
It is possible to use accrual accounting, where you book revenue when it is earned rather than when actually paid. This is complex and should not be attempted without an accountant to help you. Without taking extra steps to set up an accrual based accounting system, you should consider yourself a cash basis taxpayer.
