There is no simple way, any number of different problems can cause this.
Start with the advice you have already been given
The fact you don't have a pre-configured tnsnames.ora suggests something went wrong with your original installation. Tnsnames and a listener should have been configured for you during the install.
You obviously have little experience on Oracle, otherwise you would know where to find the documentation. Here is a good starting point for that http://www.dbforums.com/t1031644.html
My advice would be to do a de-install, clean the registry, then a fresh install and accept all defaults, preconfigured starter database etc.
Please don't email me directly with questions. I've probably just got home from the pub and cannot guarantee the sanity of my answers. In fact, I can't believe I actually made it home.