Completely new to creating databases, How do I use queries for use in forms?

I want the data to be entered through forms and perform same basic functions. These are the tables.

I've also created the design for the input screens. These can be viewed here.

I want the data to be input through forms and saved to the table once entered. Below are the functions that I want to be carried out. I'm using MS Access 2007.

(i) Items Screen

In "Items" screen, I want the type of the item to be selected through a dropbox, The types will be: Books, Stationary Items and Gift Items

(ii) Purchase Screen

Purchase Screen is completely automatically filled. ( Explained Below )

(iii) Sales Receipt

The Sales Reciept Screen basically combines the "sale" and "sale detail" tables into one screen.

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  The "date" field is selected through a calendar instead of being typed in.

  
  


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  "Customer" Field is selected through a dropbox from the customer ID's in the "Customer Table".

  
  


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  "update" saves the fields "date" "item" "qty" to the "Purchase" table

  
  


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  A subform in the form in which

  
  
  
  
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    Item is selected through dropbox from "item ID" field in "Item" table

    
    
  
  
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    "Quantity" is entered manually.

    
    
  
  
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    Price is automatically filled taking the value from "price" field in "item table" corresponding to the "Item ID".

    
    
  
  
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    Total is automatically calculated by multiplying the "quantity" and "price" fields.

    
    
  
  
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    The "Sum of Total" be calculated automatically by summing up the "Total" Field

    
    
  
  
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    "Advance" is filled in manually

    
    
  
  
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    "balance" is calculated by subtracting "advance" from the "Loan Amount" in "Customer Table".

    
    
  
  
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    "Paid fully" is a check box to indicate that all the money has been payed.