on look up, onclick of it a popup window will come, there you can select a record.
there also a picklist will be there(sometimes) where you can also change. the best example is the regarding attribute on letter.
suppose on that picklist i have 3 options as a,b,c.
bydefault a is selected and on basis of some condition i want b to be selected.
its doable and as follows.
just put the following code on load of the form.