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You said 'localhost' - that implies your database is running on your machine.
Have you installed Oracle server on your computer?
If not, I have some advice to get started after you've installed SQL Developer
Follow the below stpes
1. Download the oracle database installation document (available in PDF format on google search).
2. Download the oracle database product build from the oracle website.
3. Perform the installation as per the manual.
4. Configure the listener and then create the database using the Database Configuration Assistant or Enterprise Manager.
5. Connect the database via the SQL developer tool.
This should solve the issue for you.