Open the table in design view and select the field you want. In the general
properties tab you will see the "Indexed" property. You can change it there.
Query SQL:
CREATE INDEX myIndex On MyTable (myField)
or for unique index
CREATE UNIQUE INDEX myIndex On MyTable (myField)
or for primary key
CREATE UNIQUE INDEX myIndex On MyTable (myField) WITH Primary
You can also go into table design, locate the indexes button on the toolbar
& do it Wysiwyg