Where can I make a site ending with .com (would be great if free or super cheap) and can I show my adsense ads there ?
Making a website is a time staking process that requires you to make an outlay of some sort of funding and to think that you can get anything in this life for free is completely wrong. You can get a free domain but you can't expect for it to be a .com as to register them for a year costs upwards of $7 plus your hosting charge. As for the showing of adsense ads on the site, do you have an adsense account that has been accepted into Google?
You have a lot to learn believe me. Go to dreamhost.com to buy a .com and GOOD LUCK for the rest, I mean it!
dot com domain always cost 7 to $9 per year. You can create a free blog in blogger account and make your domain as a custom domain for your blog. Anyway atleast you need to pay for the domain name.
If you are talking about free .com domain then i think you should use free blogging service like blogger,wordpress but if you want to make a site on your own .com domain then you must have to buy atleast a .com domain name normally it costs 8-9$ but for limited time you can get it for 0.99$ from godaddy.com