There are several ways.
IMO, the most versatile is:
• in the LineItem table, create a summary field type Total of: the lineTotal field
• place this field on the PO layout.
The background is that a summary field can summarize all records in the found set of its own table, but as a related field, it will summarize all records of the related set (i.e. the LineItem records of your PO).
The versatility comes from the fact that a) you can use the same field in different related contexts, and b) you can (and will) use it on a when you print the PO using a layout based on LineItem table.
There is also a Sum() function that lets you create a calculation field for the same purpose, but you'd have to create that for every table, and you can't use it for a found set.
Once you're more advanced, you can worry about questions discussed in this thread.