SUM adds up a range and AVERAGE gives its mean. Both skip empty cells and text, so a heading or a blank row in the range does no harm.
Start typing =SU and Sheets shows a list of matching functions. Press Tab or Enter to pick the first one, or click another. Once you type the opening bracket, a help card appears next to the cell. It shows the arguments, with the one you're on highlighted. Click the arrow on the card to see an example and a short description of each argument. Shift+F1 hides or shows the card.
Ranges use a colon: B2:B13 is B2 down to B13. Sheets also lets you leave the end row off. B2:B means B2 to the bottom of the sheet, so a total keeps counting rows you add later.
Select a few numbers and look at the bottom-right corner of the window: Sheets shows their sum there. Click it to switch to the average, count, minimum or maximum.