I looked for some good videos on marching but I did not find many I wanted to necessarily endorse. On my limited search I found a video of some JROTC cadets marching who looked really sharp. Off the top of my head this video covers 95% of what you will need to know how to do by the time you graduate OTS (with the exception of parade.) The only one we did all the time which was not in the video was column of files from the right/left.
Our OTS flights had around 12-14 people and on any given day any one of us could be in charge of marching the rest of us around. There is no hard rule for this because every Flt/CC and flight themselves will decide to do this differently, but expect to do some sort of leading and lots of marching while at OTS.