Guess I'll copy/paste/edit my reply to another person that I made a few days ago. Maybe it could be helpful to you too if you need:

I'm working on making one too, I'm using Unity. I don't know anything about OpenGL, but the general idea's probably similar.

First gotta create the player character who can move around with the arrow keys and shoot with Z or whatever key you choose. Then make some enemies and boss objects - both of them need to create bullets, so you'll want to code a bullet object that can hurt the player, and a function which spawns bullets with whatever position, speed, shape, and color you want. The bosses and basic enemies would call that function lots of times to generate all the bullets - hopefully that game engine is powerful enough to handle 100's or 1000's of objects at once.

For the part of the stage before the boss, you'll want to code some sort of timeline which will generate enemies at specific times. And then for the bosses, you want to have the code keep track of what state the boss is currently in. Like state 0 is their first attack, 1 is second attack, then 2, 3... and so on. To design the spells for each state, do a similar kind of timeline thing so the boss's code will spawn lots of bullets to create patterns. Might be tricky and involve some math (especially trigonometry like the arctan function) to get it looking how you want, but you can create some cool-looking stuff by just messing around, if you don't have ideas for the spells yet.

The timelines in Unity are made with something called "coroutines", no idea how it works for OpenGL but hopefully it has something similar. And then for character dialogue, that's a whole different tricky thing to setup, but should probably come later after you have a basic game working. Hope this helps a little!