well , if you just want to show the value in textbox related to the selected value of the combo then you can do like this ,
dim con as new sqlconnection("your connection string ")
con.open()
dim da as new sqldataadapter("select val1 , val2 from table ",con)
dim dt as new datatable
da.fill(dt)
combobox1.datasource = dt
combobox1.displaymember = "val1"
combobox1.valuemember = "val2"
con.close()
now this code will get the records from the db , use this code at the selected index change event of the combo box.
textbox1.text = combobox1.selectedvalue.tostring
the val2 will be display in the textbox.
as i am typed all this code right here so may be there is some spelling mistake , please check it by your own self ,
Regards