Set the elevator trim to about 4 (in the green on the 2D Throttle Panel or the Pedestal).

Turn on the A/P at about 200 feet AGL (above ground level), but make sure that the 757 is climbing at around 2000-3000 fpm before doing so, because the MCP will switch on Speed and VS when you first turn it on. You then select VNAV and LNAV to make them active. The A/P will sometimes slow down too much (below V2), so it will return to level flight to gain speed, but I have never had it dive to the ground when doing so.