Formulas start with = — for example
=A1+B1, =SUM(A1:A10), =IF(B2>100,"over","ok").
Drag a cell's edge or use Fill ↓ to copy a formula down a column;
references shift as they move, and $ pins them (=$A$1).
Use the tabs at the bottom to add more sheets, and pull values across them
with the sheet name and ! — =Sheet2!A1+B1, or
='My Data'!A1 when the name has a space.