Before you start experimenting with security make a backup of your file!
Go to "File" -"Manage" - "Security"
Then make sure this user you created is in a privilege set you created yourself. Because you can't edit the existing privilege sets.
Then edit the privilege set.
Where it says Data Acess And Design go to "Records" and from the dropdown choose "Custom Privilege"
This means we are gointg to determine ourselves which records can be edited by the users in this privilege set.
Then select the table and on the bottom where it says "Field Access" choose "Limited" from the dropdown.
Then click on the record you want to be editable and set it (with the radio buttons on the bottom" to "Modifiable"
There is a lot you can do with the security so you better read up in the documentation.
Also make sure your user has the privilege to view the layouts (All view only) otherwise he won't be able to see anything.
If you get a big grey screen that says "No Access" that means the layout is not visible for thus privilege set.
If you can see the layout but the fields have "No Access" in them that means the records are nof viewable for this privilege set.