Create Field In The Object With Master Detail Relationship.

Hi

Read this steps:

To create the master-detail relationship field to relate custom object with the account  objectFrom Setup, enter Objects in the Quick Find box, then select custom object
Click Review.
In the Custom Fields & Relationships related list, click New.
Select Master-Detail Relationship, and click Next.
In the Related To drop-down list, choose account, and click Next.
Click in the Field Name text box and enter the field name ----.
Select the Read/Write radio button.
Check Child records can be reparented to other parent records after they are created if you want to be able to change the relationship field’s value. If you leave this box unchecked, you can’t change the value in the future.
Click Next.
Accept the defaults in the remaining three steps of the wizard.
Click Save.

Thanks