Service/Consultancies do not hold stock of anything. Whilst webERP is rich in features that assist businesses that have to deal with stock (inventory), it is also suitable for service businesses.

When an items is defined (Inventory tab -> Add A New Item) , the field

"Assembly, Kit, Manufactured or Service/Labor: "

must be defined as Service/Labour. This tells webERP that we are not keeping stock at all for this item. Stock inquiries will show N/A.
You might make an item say for "Computer consultancy" and another for "webERP support" the units might be in hours or other unit you charge for consultancy in.

Pricing for a unit of the service/consultancy item is much the same as any other item - you can price in different currencies and have prices for different customers etc. See the manual on pricing.

Often when you invoice service items you need to provide a description of what the order was for and this can often be several lines - you will not wish to go through changing the item description every time - it is too short anyway and will not show correctly on old invoices that had a different description. The answer is to use the feature that allows free-form narrative against each line item ordered/invoiced. This feature is accessed from Setup -> Configuration Settings