Ok, that means the command is working properly but the listener service isn't set up yet? Have you finished configuring the Net8 config? If not, follow these steps:
Open the Net8 Configuration Assistant. From the Welcome screen, click Next and then select "No" at the next screen to manually create net service names. In the "Naming Methods" window, accept the default value (Local) and then click Next. You will asked which version of the Oracle database or service you want to access. Select Oracle8i and then enter the name of the database (the one you set up Oracle with) It will be in the form of oracle.world, where you can replace oracle with your own database name (SID)
At the next screen, select TCP and enter 127.0.0.1 in the IP address and select the port as 1521 (default) Then, you will be prompted for a login test. Select "Yes, perform test" and if everything is working properly, you will have a successful login test. If this works properly, click Finish and then exit and you should be able to connect to your database via SQL*Plus.
EDIT: Are you sure you selected "Install as a service" when prompted to do so during the installation of Oracle8i's TNS service when you configured the database name and the way it started and stopped, etc.?