The priorities are to bring about a broad ceasefire, stop the Islamic State group (IS) and increase aid, he said.
The announcement came as 23 people were reported killed in an attack on a rebel checkpoint in Aleppo.
A suicide bomber driving a fuel truck targeted a checkpoint run by the rebel group Ahrar al Sham, the Syrian Observatory for Human Rights said.
Four of the group`s commanders and four civilians were believed to have died, according to the Observatory, a London-based monitoring group. Dozens more were wounded.
More than 250,000 people have died in Syria in the almost five years of fighting since opposition protests against the rule of President Bashar al-Assad began.
Mr de Mistura said the "proximity talks" would be indirect to start with, with negotiators ferrying messages between the sides.
The talks had been delayed by disputes over who should attend, he said, but he added that he expected invitations to be sent out on Tuesday.
Participation by IS and the al Qaeda-linked Nusra Front had been ruled out, he said, but discussions about other rebel groups were continuing.
Turkey has warned that participation by Syrian Kurdish forces, which it considers to be "terrorists", could cause the talks to fail.
Mr de Mistura said the first phase of negotiations would last between two and three weeks.
He warned the talks would be stormy, with "a lot of posturing, a lot of walk-outs and walk-ins".
Issues of governance, a constitutional review and future elections would also be on the agenda, he added.
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