Macro To Produce Purchase Order Data

i have a purchase order sheet

firstly i have attached said spreadsheet
1st sheet is named purchase order
2nd sheet is named purchase order numbers

upon opening the spreadsheet i would like on sheet purchase order to automatically input the next available number from column a in sheet purchase orders ( in this case next available is st010 and put that into cell k12 on sheet purchase order

i then want to enter the relevant data in sheet purchase order cells
b16 , b23 , g23 , i23 and d28

i then want to click a button with macro attached to transfer that data in to sheet purchase order numbers