Hello.
There are several ways how to do it. If you mean it for CMSEditors, you can set "Starting alias path" in CMSDesk -> Administration -> Users - edit -> General section.
If you mean it for public users in some existing role, you can take an inspiration from following
KB article.
Or, you can modify Logon form web part´s code-behind, or place custom web part (user control) or given page and do it with custom logic. All you need is to check user name, filled in logon form web part, or take it from CMS.CMSHelper.CMSContext.CurrentUser.UserName property, and write some decision-making logic, which will redirect given user to required URL. You can use UrlHelper.Redirect(string URL) method for this.
Best Regards,
Radek Macalik