Hi Sindhu,
You can create custom tables by clicking Enter Data Button under the Home tab.
Click on * to add new Columns/rows.
Now enter the table name and Double click on the columns to add columns.
You can also add data by Double-clicking on the data cells.
The Column that is created in already existing tables can be added in the new table by merging or creating a relationship between them. Also, Learn How to create a final table based on the Joins of two tables in Power BI.
Hope this helps you.
If you are interested in learning Power BI, check out Power BI Course Content now!