PROGRAM DOES NOT KEEP ACURATE TRACK OF INVENTORY , WHAT CAN I DO ?

Typically the program does buy and sell inventory just fine.  Using inventory parts to make another part that is held in inventory is something QBO is not designed to do.  You really need to explain what is not working, how you are doing things.

inventory has to be purchased using the item details part of a purchase, if you do not see a section titled item details, then turn it on in company settings.  When you click on that title a table opens where you list the items purchased, qty for each, and total cost for that qty.  That stocks the item with qty and cost.

Then you use that item on a sales receipt or invoice to sell it.  When it is sold, the qty will go down and the cost will post to COGS

Inventory items have three accounts 
expense = COGS
income = your sales income account
asset = inventory asset