If you're new to Access, the easiest way would be to simply source your combo box list from a table and include the value you want as your default as a record in the table. Then, go to the Default property of the combo box control and set it to that value.
I know that is not the "best" way of doing it, but it works every time. For all my databases that someone in the various work units will maintain, I try to keep it all extremely simple.