Last Name and First name are not unique fields in AD. If you want info on the logged in user, you can use the loggeed in user which gives you the SAMACCOUNTNAME. That is unique, so you can search on that.
Otherwise, you'd have to generate a list of users with the last name you are looking and have the user select the right one. Then you can retrieve all the info about that particualr user using Get User Details.
rob