Numbers: Click the Format icon (the paintbrush) in the toolbar, then select the Cell entry in the resulting sidebar. Select the option (Automatic, Number, and so on) you want to use from the pop-up menu. You may need to set other values: For example, if you choose Numeral System, you'll need to set values for Base, Places, and how to represent negative numbers. (Numbers also includes special number formats such as Slider, Stepper, Pop-up Menu, and more; these can be used to create intuitive data entry forms.)
Sheets: All number formats can be found in the Format > Number menu; each formatting option appears in its own submenu. As in Excel, you can create custom number formats that mix text and numbers--but you have to find the option first, as it's buried in the Format > Numbers > More Formats submenu.
2. Merge Cells
Another useful formatting trick is to merge cells. Merged cells are what they sound like: two or more cells merged into one. This is a great way to center a header above a number of columns, for example. Merged cells are a powerful way to get away from the strict column-and-row layout of a typical spreadsheet.
To merge cells, you want to have a value only in the first cell you intend to merge, as values in any other cells will be wiped out by the merge. Select the range of cells to merge, by clicking on the first cell (the one containing the data) and dragging through the range you wish to merge.
Excel: Click the Merge entry in the Home ribbon, and then select one of the Merge options that appear in the pop-up menu--Merge and Center is what I use most often.
Numbers: Select Table > Merge Cells.
Sheets: Select Format > Merge Cells, then choose one of the Merge options, such as Merge Horizontally.
You can also merge cells vertically, which can be useful in tables where you have a parent cell (Salesperson, for instance) that contains multiple rows of data (for example, Product Sold and Units Sold).
3. Use Functions
You probably already know how to use basic formulas to do basic arithmetic on cell contents. But functions, which let you manipulate text and numbers in many other ways, are how you really unlock the potential of spreadsheets.
If quantity mattered most, then Excel would win, with (if I counted correctly) 398 unique functions. Google Sheets comes in a close second with 343, and Numbers has 282. But the total count is irrelevant, as long as the app has the functions you need.