You can put a text ad. Highlight the text you want linked and then click on the chain to insert or edit a link. You should not be posting banners or more than one of your own affiliate links. Most people are in the HubPages Earning Program or they use adsense. So they make money with the ads HubPages displays automatically.
If you put an ad from the company you want be careful. It needs to be related to the hub. Generally people write hubs to provide content and HubPages looks after the ads. If you don't have an adsense account yet write more hubs before applying for adsense.