You can complete the form as soon as you have your travel dates as once completed and approved they are valid for two years, or until your passport expires, whatever is the sooner.

 

If you need to change your passport before you travel do that first, otherwise you will have to re-apply for an Esta Visa.

 

This form MUST be completed for every member of your party, Please Note, If you haven't completed the ESTA before travelling, or you did but were declined, you may not be allowed to board your outbound flight, if you think you've been lucky enough to make it across the Atlantic, think again, Unless  the US Border Protection Agency encounters a serious computer malfunction you're guaranteed to be turned away and sent back home, this will be at your own expense, this includes everyone: babies, children and the elderly

It's worth remembering that anyone can apply for ESTA on your behalf, and, although the Department of Homeland Security recommends giving at least 72 hours for a decision to be made, often it only takes seconds, however, if  you book and pay for your flights first and are rejected you'll lose the cost of your flights, and any associated expenses such as hotel bookings and car hire (if they're non-refundable) as insurance providers will almost certainly not cover you for this.

Also note there are a large number of companies that are very happy to charge you up to £50 (for each form) to do this for you, Do not use them, the form is very simple to fill in and will cost you $14.00 per person (aprox £9.00), a point to remember here is that these companies have got to ask you the same questions that the Esta form asks in order to fill the form out for you, and you would have to type those answers into an e-mail, simply type them in the Esta form and your done.

 

Looking in google or other search engins will bring up a large number of these companies, simply click the  Esta link above or below to visit the correct Esta Website and fill in the online form to apply for your visa.