I Have These Tables:
Case, Agents, Forwarders, Orders.
1 Case Has Each Time 1 Agent, Many Forwarders And Each Forwarder Many Orders
1 Agent Many Forwarders
Can Someone Suggest The Best Way To Link Them?

Any tables are linked on Primary key to foreign key. So each table should have a primary key (preferably an autonumber). Related data are represented by storing the primary key value as a foreign key.

From your description, AgentID would be a FK in your Case table and ForwarderID an FK in your Order table (assuming an order can have only one Forwarder). You would need to add a CaseForwarder table that would have both the CaseID and a ForwarderID as FKs.