You can set the default options for comments under Settings ->Discussion in your dashboard. First you can require that visitors be logged in to comment (either wordpress, facebook, twitter or google+) or to leave an email before they're allowed to comment. You can disable both of these, but that is not advisable as it can lead to a significant increase in spam: http://en.support.wordpress.com/who-can-comment/
You can also set whether you need to approve all comments or only a person's first comment to allow them to post comments. If you pick the second option you will only need to approve them the very first time they comment and thereafter they'll be able to comment on your blog whenever they choose, unless you change the settings. You can also deselect both options, i.e. have no comment moderation, but as above this is not advised: http://en.support.wordpress.com/settings/discussion-settings/#before-a-comment-appears
As for the advertising: you can't and doing so will get your blog suspended. WordPress has the WordAds program for which you can apply if you have a custom domain, but no other user-added advertising is allowed.
If you want to earn revenue from advertising through your blog you'll have to purchase your own domain and hosting and install a WordPress.org website. Then you will be able to add whatever advertising you want.