At a summit in Latvia, he will outline changes he wants to see, including restrictions on benefits for migrants reports BBC.
The prime minister said he expected “lots of ups and downs” but was focused on giving people a “proper choice” in a referendum due to be held by 2017.
EU leaders are in Riga to discuss relations with ex-Soviet states.
Mr Cameron said this issue was very important in its own right but that the gathering was also “an opportunity to start some of the discussions about reform of the EU”.
“All I will say is that there will be ups and downs,” he told reporters as he arrived at the conference.
“You will hear one day that ‘this is possible’; the next day something is impossible.
“But one thing through all this will be constant and this is my determination to deliver for the British people a reform of the EU so they get a proper choice in that referendum we will hold before the end of 2017, that will be constant.”
The prime minister is meeting his European counterparts for the first time since securing his re-election and a majority Conservative government.