It all depends on the number of steps. If there is an odd number, you must start with you right foot; however, if there is an even number of steps, you must start with your left foot, only if you approach the ambo from the left side. If you approach the ambo from the right side, then reverse the order of which foot goes first. This also applies to which hand you use to turn the pages of the Lectionary. First, do not use your feet. When turning the page of the first reading, you must use the hand according to which foot you used first to climb the steps (right or left). For the second reading, you must reverse that order. If you turn the page to the Gospel Reading for the priest, either hand is appropriate, unless you are left-handed. In that case, you are to refrain from turning the page to the Gospel Reading. If the deacon or priest will be approaching the ambo with a Book of the Gospels, be sure to close the Lectionary with your left hand only, do not let your right hand touch it; then, when closed, use both hands to place the Lectionary on the ambo shelf face up if you are right handed or face down if you are left handed.
Gee, these rules are simple. I do not understand why others here did not know this!