This will be your first blockade type of level, which will usually put you in a small area and pit you against some enemies. For 1-A, you will be facing off against two Chargin’ Chucks. These are simple enemies, with their only attack being to charge straight at you. As you noticed, Chargin’ Chucks are wearing helmets, so they will take more than one hit to defeat; think of them as similar to Koopa Troopas in a way.
The first hit against the Chargin’ Chucks will remove their helmets, after which, they will need another bop to the head to get rid of them for good. Note that you’re not restricted to just jumping on their noggins, as any power-up can also put them down, such as a Fire Flower or a Super Bell. Upon defeating both Chargin’ Chucks, you will be rewarded with some coins, as well as a Green Star. Upon collecting the Green Star, you will be finished with the stage and granted access to the eastern half of World 1.