(see guidelines for overcoming access barriers)
The tab key can be used to move through all web pages by default but you can use the tabindex attribute to control where the tab key goes.
Most standards-based websites don't need to include tabindex for their links. When they are included along with other techniques such as skip to content links, they can hinder functionality and usability. This is why although it is a WCAG 1.0 Priority 3 recommendation (checkpoint 9.4) the W3C WCAG 2.0 Draft (30 June 2005) recommends against it.
If - despite your best efforts - the natural tab order is not a logical tab order you should use tabindex. Assign the tabindex attribute to a link. For example, using the following code, the link to the home page would be the first element the tab would move to.
<a href="home.htm" tabindex="1">Home</a>