new to js, need some help....

I have a form that has 2 fields... accesslevel and url

i need a function that says if accesslevel = 0 then write user/
default.asp in the url field, or if accesslevel = 1 write manager/
default.asp in the url field....

can someone help me get started on this? the accesslevel field is a
select list so i assume that there can be a function that is called in
the onchange event... i am having trouble with the syntax bc i havent
messed with js that much...
.