I suck at Lattes and Espresso stuff

So I’ve worked 3 days and Dunkin so far, and I suck at lattes. What I suck at the most is the freaking espresso machine. I’m pretty sure small espresso is small drink and Medium espresso is medium drink and so on. But what’s the deal with the double shot triple shot stuff? And how do I tell when a drink needs that? Cuz it doesn’t say Double shot on the tickets. Also how long did it take the workers here to learn Lattes and all the espresso stuff? Cuz I feel like I’m majorly sucking at it