Move left and press X to crawl under the truck. When you get control again, pick up the AK from the dead soldier near you. Head forward for another part of the cutscene. The Gekkos show up; stay to the right side of the screen and move quickly. Use X + direction to dodge and roll if they kick at you.

There are several shortcuts - a pile of degree to crawl under, and a window to roll over. Head towards the objective point on your radar. After the cutscene, you get the briefing and the proper introduction to Act 1.

Metal Gear Solid 4 Walkthrough