You don't. Webmail is not the same thing as a word processor like MS Word, Mac Pages or Open Office. There is no provision for using tabs. So you need to use whatever word processing program you have, write your document and then email it as an attachment. You could do the same thing using one of the online document utilities like Google Docs, or Pages in an iCloud account.
That said, you can make columns or the effect of a tab by using the spacebar on your keyboard. I just tried it sending an email from my Comcast account to my Yahoo account and it worked OK. I put 5 spaces between words using three lines and the spacing held both in Yahoo and in Mac Mail. The only thing with this method is that if you are composing more than 3 or 4 lines it gets tedious.