How can I get vertical text boxes in "Numbers" App?

Vertical "Text boxes" are easy.

Paste your text into a Text box, using a mono-spaced font (such as Andale Mono, shown here), Grab the handle at one end of the box and drag it toward the other until the box is narrow enough to force the letters into a single column. (Won't work with a proportional spacing font—a column wide enough for a W is wide enough to hold 'it' and other similar combinations.)

If you are actually asking "How can I put vertical text in a spreadsheet table cell," see the post linked in Wayne's message.

Regards,

Barry

Or if you already have a text box and you want to rotate the text, you can do this:

SG

You don't. There are only work-arounds. See Ians post on this from Yellowbox and SGIII:

Rotate cell contents in Numbers 3 and 4: Workaround