A pretty good, step by step guide is here.

1.) Get information from your tnsnames.ora file.
2.) oracle12.jar can be found here, though you might want ojdbc14.jar, also on that page.

Here, we can find some important things. Match your tnsnames.ora file and note the following details like me:
Database-name : XE
Server / Host = lordamit-laptop
Port: 1521

Remember, these are case sensitive. close the tnsnames.ora file. Now, once again. get back to squirrel. From the left menu, select Aliases. Click on the plus button to add a new alias.

now, make the following configurations [in SQuirreL]:

Name: myHR (change it if you want)
Driver: Oracle Thin driver
URL: jdbc:oracle:thin:@lordamit-laptop:1521:XE (replace it with the info you got from tnsnames.ora)
Username: HR
password: iit123
now, press ok. It will be saved. You can now press Connect to connect to your oracle