I have to pick document no from dfkkop and put into table dfkkinvdoc_i(field opbel(document no))
From dfkkinvdoc_i i have to pick INVDOCNO and pass it to table dfkkinvdoc_h
and from dfkkinvdoc_h i have to pick purchase_order.
Yes, this is your job, good luck !
Also, "need your help" is not a good title because it tells us nothing about your specific request
Basically, re-read your course materials on reading data in Abap, SELECT, ABAP SQL Statement
(Or explain what your exact problem/need is, as it looks like you already got all the information you need to start coding)