Not sure where do you stand on this process now. But for basics to start with read below links
1) SAP archiving process-Simple steps
Below will be your strategy
1) Find with help of Basis team table size
2) With help of basis team monitor the growth of table and find two 20 tables growing at alarming rates
3) Finalize table list
4) Find SAP archiving objects for corresponding tables
5) Archive them in test system and validate