Need help with a US waiver visa?

ok im from Spain and im traveling to US next june 1.but i notice that i need to fill up online the waiver visa.. my question is..do i need to fill up now or wait 15 days before my travel? i really confused coz im doing that in the airplane (waiver visa) but now i need to do it online.. is this the correct website to fill up? https://esta.cbp.dhs.gov/esta/esta.html?_flowExecutionKey=_cDFD1497D-CD7C-07DF-E4D0-2B3E1E214F4E_k83D6CDB7-313B-9679-F6A1-C8FB699E29D4... please help me thanks in advance