first off you would need a minuim off 50mb but i would recommend 150mb, you also need php hosting, hostingdepartment.net is recommend for hosting. then you need your forum software if you cannot afford to purchase any software like vbulletin i would recommend phpbb
You won't need that much bandwidth in the beginning. How do you plan to populate your forums though? Postfusion can help you.
I'd use a free forum script to start with. Then if it gets popular, you can move it to your own domain.
Mybb or phpbb is right for you at the beginning.If you wish to start a forum with uploading facilites you need to approach freeweb7.com or 12gbfree.They are providing good space and b/w for free.You can approch a premium hoster if you've cash