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.
But it is the first opportunity Mr Cameron has had to discuss his proposed reforms since winning a second term in Downing Street.
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."
Context of changes
The prime minister is meeting his European counterparts for the first time since securing his re-election and a majority Conservative government - at an event that will be dominated by the EU`s relationship with Russia and its neighbours.
Legislation paving the way for the UK referendum - which was promised in the Tories` election manifesto - is expected to be published next Thursday, the day after the Queen opens Parliament.
BBC deputy political editor James Landale said Mr Cameron would use the summit to sound out other European leaders about what they might be willing to accept before the start of "intricate" negotiations in the coming months.
While other EU leaders, he added, were "scared witless" by the possibility of the UK leaving the EU and we were willing to listen and consider a deal, there were some things - such as changes to freedom of movement rules - which were "sacrosanct".
Mr Cameron has not revealed the full details of what he is seeking from any changes, but he is expected to demand an opt-out from one its core principles of forging an "ever-closer union" between member states.
He will also try to get more powers to block or opt out of new EU laws, and for restrictions on welfare benefits for migrants until they have lived in the UK for four years.
In a speech on Thursday, the prime minister said welfare changes would be an "absolute requirement in the renegotiation".
David Cameron is ready to start renegotiating the terms of Britain`s EU membership ahead of a referendum, but what does the UK prime minister want from Europe?
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