From the starting position...
...turn around to face the ramp Lara just slid down. Go into the opening in the left (northwest) corner.
At the end of the short passageway, where the ceiling is higher, roll or turn around...
...and climb onto the ledge above the passageway you just came through.
Crawl up the ramp and through the low opening.
Pull up onto the ledge directly ahead.
Climb two more stepped blocks toward the base of the right-hand ramp. (The room with the big statues is below on the left, and you can see light coming in at the top of the ramps.)
Climb onto the base of the ramp and enter the alcove on the right. This is SECRET #1.
Pick up the SHOTGUN (loaded with 6 regular shells) plus a box of wideshot ammo.
Climb back down the blocks to the northwest corner ledge.
Then slide the rest of the way down to the starting point.