I've set a table which contains information about customers in my shop. I then used this table to create a subform within another form. The subform displays the customers information, in the form I've also created a text box. I want to be able to type a customer's code into the text box and for the correct record to be displayed in the subform.
Can anyone help?

Just set the Master/Child links properly in the subform and you will be good to go. I might also suggest that you put the Customer's code in a combo box as it is easy and no chance of typing the wrong code.