Once you create a new ad campaign, you can activate it by going to Campaign > selecting the campaign under Campaign List > sliding the Status bar from Inactive to Active.
If you activate a campaign, all ad groups under that campaign will also be activated.
If you only want to activate some of the ad groups, then you select the campaign under Campaign List, where you will see a list of ad groups > slide the Status bar of the selected ad groups from Inactive to Active.
You will receive a campaign-activation notice when the campaign is approved and goes live.