You can visit Death Peak early in the game, and even find a Power Tab, but you won't be able to get very far until much later. Once you have the C.Trigger in hand, it's time to climb the mountain.
The Nu in the Keeper's Dome will send off several Poyozo Dolls to help you. The first turns into a tree you can use to block the wind while climbing the first area. I find it useful to stop running while behind the tree, so I don't accidentally sprint around it.
There are three Lavos Spawn on Death Peak which serve as Bosses. They are all the same, and the same strategy applies to each: do not hit the Shell! Nasty counterattacks await you if you do. Concentrate on the Head with whatever attacks you have.
Once past the third Lavos Spawn, you'll reach the summit, and Crono will rejoin the party.