Your first major option for watching tv online is Hulu. Hulu is located at www.hulu.com and provides numerous shows and movies in great quality. Hulu has ads placed at intervals during their videos but often they provide the best quality for online television.
Another option is www.tvshack.net . This site is one that compiles links from a bunch of other video hosting sites and categorizes them into shows and movies. The shows are categorized even further by season and episode. Even though this one is not as legit as Hulu you can find a lot more shows that are often in good quality.
A third place that you can find tv and movies online is youtube. www.youtube.com is a great place that host numerous online videos. Even though most are just clips you can often find full shows and movies on the website. Youtube also provides a lot of user created content and so you can find some video gems that are just as good as your normal tv experience.