A table is the basic set of fields containing data, you already have one table, most likely called Members, which has fields like name, address etc? A Portal allows you to display and edit records from one table on the layout of another table
Click on File | Manage | Database, which will display the table(s) in your current file, and their fields and what relationships are set up. Have a look at what tables/fields/relationships already exist in your database. From your previous post, you appear to have a unique, auto-enter serial number in your Members table - good.
This number can be used to link related records in another table..
Again click on File | Manage | Database (unless you're still in there). This is also where you create additional tables for your database. Select the Tables tab, type a new name (maybe Donations) in the Table Name at the base of this window, and click Create
select the Fields tab, select your new table in the drop down list of tables, start creating fields in here as I indicated in previous post to the other thread
When you've got your new table set up, click on Relationships. Your two tables will be displayed as small windows. Select Add Relationship (second icon from left along the bottom), In the left part of the window select your existing table, and the ID field, in the right part select the ID field in your Donations table, click the 'equal' sign in the centre, and check the box "Allow creation of records..." under the Donations side. Click OK a couple of times to get back to your original layout. Enter layout mode, check that it is displaying records from your first table, Insert a Portal, and when prompted, "Show related Records from..." Donations (or whatever you've called your new table). Select the appropriate fields to display in your new portal.... you can add donations via this portal to existing members
Buy a book ... watch a tutorial .... open one of the many starter solutions supplied with Filemaker,,, good luck