Mr Cameron, whose main address is on Thursday, said the Tories needed to show they "deserve to go out and win" the next general election and deal with Britain's "severe" problems, including the financial crisis.
Warning members against complacency he said: "We need a strong and positive mandate for the changes we know our country needs and we know people are crying out for.
"This is not some week of celebration, not some week of pointing out Labour's failures.
"This is the week we should square up and look at the British people in the eyes and say we know how deep the problems are, we know how difficult some of the solutions will be.
"But we are ready together with you to take this country through some tough times - but to better days ahead."
In another announcement, the Tories said any business created in the first two years of a new Conservative government would get a tax break for the first ten staff it takes on.
The measure could create 60,000 new jobs, the party said.
Labour ministers have called the welfare plans "callous" but Mr Cameron told the BBC it was not about trying to reduce people's standards of living.
He said for those who "genuinely can't work, who are disabled, we as a compassionate society should look after them, should help them, and should pay their benefits".
But he said others could work and would be given "tailored help and support" to get work.
The Tories plan to scrap Labour's New Deal back-to-work programme in favour of more personalised help, offering training courses for those unemployed for more than six months.
Benefits would be cut if people refused to take part and private training firms would be used to assess all those on what used to be called incapacity benefit and to prepare those capable of working of looking for a job.
They are simply rehashing Labour policies without the investment needed to make them work
The Tories say the £600m cost of the policy would be covered by savings made from moving people from incapacity benefits to jobseeker's allowance.
Meanwhile the government has announced a number of new policies aimed at getting more than 750,000 people back into work by the end of next year.
People out of work for more than six months who have turned down work experience, support or training will be required to take a work placement as a condition of receiving their benefits.
Work and Pensions Secretary Yvette Cooper rejected the Conservatives' welfare-to-work proposals as "a carefully packaged con".
"The Tories still want to make cuts in a recession that would destroy jobs and, according to respected economists, would actually push unemployment up to five million," she said.
"They are simply rehashing Labour policies without the investment needed to make them work. The Tories don't understand that you can't get people from welfare into work if you destroy the jobs for them to go to."
But the Conservative plan was welcomed by Richard Lambert, head of the CBI business organisation, who said: "The struggling economy and rising unemployment demand fresh thinking from all political leaders and we like the look of many of these suggestions."
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