Hi! When I was making the game I thought the charge jump was fairly intuitive, but maybe it's not as well explained as it could be lol
A charge jump is performed by charging up your attack, aiming downwards, then firing off the charged attack. The huge recoil of the charged attack boosts you upwards, acting as a sort of second jump if you do it mid-air. It's this game's version of a double jump.
Here's a gif showing how to cross that gap, hope that helps.
Also, yeah, to find the first boss you need to do some backtracking, not super intuitive... I'd probably add some objective markers on the map screen or something if I ever update this game in the future .:)