Not used adsense before, was wondering can you select what ads show on your site, or is random, I mean I know if your site is related to movies for example you will show sites that supply movies, but what I mean is can you individually select what companies ads you show? Thanks
No you cannot pick what companies you want to show. Adsense is a contextual network, meaning whatever text and meta tags you have on your page... say for example "jennifer lopez" then those ads will come up related to her. BUT, if you have more ad blocks on your page then google has inventory for relating to "jennifer lopez" it will go ahead and start displaying ads related to her... example: "movies" "music/cd" or even other celebrities.
google makes it simple. there is no category to select. ads will be displayed based on your site content. you can install Adsense Preview Tool (just search for it) and see what types of ads will be served on your site.