This is an idiom that means "began to act." Whether Peter was sitting or standing when he decided to act is not important. Alternate translation: "started out" (See: [[rc://en/ta/man/jit/figs-idiom]])
Peter had to bend over in order to see inside the tomb because tombs cut in solid rock were very low. Alternate translation: "bending himself at the waist"
"only the linen cloths." This refers to the cloths that had been wrapped around Jesus's body when he was buried in [Luke 23:53](../23/53.md). It is implied that the body of Jesus was not there. Alternate translation: "the linen cloths in which Jesus's body had been wrapped, but Jesus was not there" (See: [[rc://en/ta/man/jit/figs-explicit]])