Before doing anything, make backup copies of your web directory AND your DotNetNuke database. If you do, you can always return to this starting point.If none of your known username/password combinations works, then I would guess that you don't have the right machineKey settings in your web.config file. Keepking a good copy of the machineKey settings is an absolute necessity when working with DotNetNuke.
If you have a copy of the web.config file that was created when your first started the site, you'll need to copy those settings to your current web.config.
If you don't have those settings, you might not have any choice other than to start over.
If you start over, you'll want to keep a copy of your current DotNetNuke database file. You may be able to copy data from that database to your new one, once you get the new one working.
Create a new database, DNN_NEW, and then edit your web.config file to use it and not the old one.
This is probably a good time to suggest -- before you do anything else -- that you take a deep breath, relax, and spend some time reviewing some of the really great tutorials that DNN Creative provides. I would suggest that you review all of these before doing anything else:
An Introduction to the Principles of DotNetNuke
Quick Start Guide to DotNetNuke
Basic Introduction to Working with Pages in DotNetNuke
How to optimize an image before uploading it to your website
Working with Images in DotNetNuke
Beginners html guide for DotNetNuke
How to adjust your Nameserver settings
Installing DotNetNuke 3.2 to a website hosting providerThere are other tutorials that will be useful, too. Browse the DNN Creative site to find more with titles that seem appropriate.