You definitely should invest in some training material. You'll need to research the options and pick what looks best to you as the prior skills you already have will determine the 'best fit' for you.
First, make a back up copy of your file. That way, if you make a mistake and have trouble getting your file open with full access, you'll have the option of discarding this copy and trying again with the back up copy.
Next, select Manage | Security.
Select the existing Admin account, click Edit and enter a password in the password box. Write down this password on a piece of paper and lock it in a safe or equally secure location. If you lose or forget this password, you will not be able to make any further changes to your database. Click OK twice to close Manage security. Enter Admin and your new password when asked to do so. Now close and re-open your file. When you open it, you should be asked for an account name and password. Enter Admin as the account name and your password. Keep in mind that the password is case sensitive, you'll need to capitalize any letters just like you did when adding the password in Manage | Security.
You've now locked up your file so that no one else can open your file without first getting the account name and password from you. Next step is to add a second, limited access password that does not allow a user to delete all records from the database.
Return to Manage | Security and click new. Enter an account name and password. In the privilege set drop down, select the "data entry only" option. Now click OK again, entering your Admin account name and password when asked to do so until you have exited manage | Security. Now close and re-open the file again, but enter this new account name and password. You should find your menu options are more limited, you cannot enter layout mode and you cannot delete records. If you want to allow users to delete individual records, you can re-open the database with the admin account and add a button to your layouts that runs a script to delete the current record. If you select the "run with full access privileges" option for this script (This is a check box in the script editor), you'll be able to open the file with the limited access account and delete individual records but not select "Delete found set" or "Delete all records".
You can find out much more about these options if you look up Manage Security in the FileMaker help system.