If you're creating a new campaign, it will automatically advertise on the content network and search network unless you tell it not to. If you've already got a campaign on search, but not on the content network, go into the campaign settings, and tick the box. You can also untick the search network if you want the content network alone. You can change the bids on the content network at Adgroup level - this can be done in the same way that you change bids on the search network.