Representatives from more than 50 governments and international organisations, including US Secretary of State Hilary Clinton, are due to attend the London Somali Conference on Thursday to discuss how to tackle "piracy, terrorism, conflict, poverty and famine" in the nation.
The Prime Minister told representatives from Somalian community groups that it was important to be "realistic" about how much progress could be made at the one-day event but insisted he had "great expectations".
At a roundtable discussion with the Somali Diaspora in Downing Street today he said: "The strategy is to try to get the whole of the world to get behind the efforts of the Somali people themselves are making to build a stronger, safer, more prosperous country."
He added: "I just feel passionately that this is a country that has had such terrible drought, famine and war that we really ought to, over the world, be doing more to try to help.
"Let me absolutely stress this is not the West have a conference on Somalia and telling the Somalis what to do, it is actually just recognising there a huge amount of effort going on inSomalia to help rebuild the country away from the conference and actually, what the world needs to do is help that take place and to grow. That's what really lies behind this conference.
"I think we have to be realistic. It's an incredibly difficult situation.
"I have got great expectations and great hopes for what I think we can achieve in lots of different areas but I think we have to be realistic about what we can do in one conference."
Mr Cameron added: "In the end, it's about giving everyone in Somalia, particularly young people, some hope of a job and a voice and a future."
UN Security Council experts are discussing a strategy that would see the African Union force in Somalia extended from 12,000 to about 17,700 troops.
It is hoped a draft resolution aimed at breaking up the "business model" used by pirates in Somalia could be adopted on Wednesday.
Earlier this month William Hague warned more action was needed to tackle the terrorist threat from Somalia when the first British Foreign Secretary to visit the strife-torn African nation for 20 years.