I'm trying to replace the bits inside my toilet. The water supply line is stuck where it goes into the toilet, The plastic connector seems "glued" to the fill valve assembly.
How can I get it off?
If the line is made from solid copper (often chrome plated), it sometimes has a bit of tension in it, making it appear to be stuck to the fixture. Try gently pushing it in different directions while pulling down.
If the line is a flex line, it may be stuck with a bit of mineral buildup; try gently wiggling it back and forth.
If you're not comfortable with the amount of force needed, it may be too much. Consider calling a pro rather than breaking something.
Everything stated above both in comment and answer offered by TomG is true, but I think that you really should find the main valve and close it while you are performing any intervention with the pluming. Also if it is metal you can apply some products which help when screws or valves are stuck. If it is plastic, try gently unscrewing it; even if you break it wouldn't cause bigger problems because you have closed the main valve.
Worst case you can cut the plastic nut off using a Dremel and a cutoff wheel. It likely isn't glued but if you are having this much trouble getting it off, it's unlikely you will want to reuse it anyways. Same goes for the supply line, new braided lines are cheap so if necessary, just destructively remove it.