Why are you joining the 2 tables and what data does each table contain? For example, if table1 contains sales data - and has a column named StateID, and table2 is the State column and contains StateID, StateMnemonic, StateName and you need to return the Mnemonic (e.g. TX, CA, WA) instead of the ID then you would join to the State table using the StateID.
On the other hand - if the StateID were actually the mnemonic and you only needed to return that value then you would not need the join.
Further - if table1 is for Claim data and table2 is ClaimDetail and you have the columns OrgID, GroupID, ClaimID, (other columns) in the claim table and the ClaimDetail table has OrgID, GroupID, ClaimID, DetailID, (other columns) then you would need to join on the 3 columns from the Claim table to get correct and accurate results.
If your systems does not have any PK/FK definitions - hopefully there are indexes available, including a clustered index. You might be able to figure out the PK based on any unique key constraints or indexes.
One final note - do not trust the ID column is the *same* domain in every table. In many systems this column is added to every table as an identity and has the same name in other tables - but is NOT the same value in those tables. Hopefully you don't have a system that was designed that way.