A percentage in a spreadsheet is just a fraction shown with the percent format: 0.25 displays as 25%. So you write the division and let the format do the rest.
- Share of a total: part divided by total,
=C2/C7. When you fill it down, lock the total: =C2/$C$7.
- Percent change: new minus old, divided by old,
=(C2-B2)/B2. The brackets matter; without them Sheets divides first.
- Raise by a percent:
=B2*(1+$F$1) with 5% in F1. Cut by a percent: =B2*(1-$F$1).
Keep rates in their own cell and refer to them, so a new rate updates every row. Type a rate as 5% or 0.05; typing 5 means 500%.
To show the result as a percent, use the % button in the toolbar or Format > Number > Percent.