The Charge Worm (Officially known as Deorem according to lore but nobody would actually know that) is the first boss you will encounter in Zero Misison. He spawns when you enter the room where you collect your first missiles, but only after you've collected the missiles. If you fail to defeat him, which can happen if you miss all your shots or run out of ammo, he will leave and return to fight you right before the first Energy Tank.
It takes 3 missiles to kill this worm and it rewards you with the Charge Beam after defeating it. Normally you will need to wait for it to open its eye and shoot it one at a time, and hoping it doesn't throw spikes at you because they waste more time to watch the spike animation. Most runners will attempt a strat called Doubles, which involves shooting a missile and immediately jumping to fire a 2nd missile so that they both overlap and hit the worm's eye on the same frame. There isn't much of a cue for this trick since it's all timing, the best advice is to practice with save states until you figure out a timing that works for you, and then continue practicing that timing to build consistency.
One of the more subtle timesaves in this fight is purposefully moving to either the left or right side to lure the worm down at an angle. It moves faster to compensate for the longer distance, and it's actually faster to have it move that way than it is to let it move straight up and down.
There are two opportunities to hit the double, either during the first round or the 2nd round. If you miss both rounds, the third missile will kill him anyways so there's no reason for a double. Attempting and failing the double both times only costs exactly 5 missiles, so there's no reason not to go for it, but it's also not worth resetting your run over missing it unless you're going for top times.
If you hit the first double, you get the opportunity to do a dboost into the worm to kill it even faster. You need to hold angle+down and position yourself near the right side of the worm after hitting the first double, and as it's lowering towards you, you run into the worm to get iframes and shoot its eye right as it opens.
Another important note about this fight is if you miss the first double, you will want to wait until the worm rises all the way back to the ceiling before shooting the 2nd missile at it. If you shoot it while it's still rising, you are pretty much guaranteed to get spikes and waste more time before the 3rd shot. If you wait until its on the ceiling, you still have the possibility of getting spikes anyways, but it's a much lower chance.