FIX 4.3 implementation

Imported from previous forum

[ original email was from srinivas eadha - hesrini@yahoo.com ]
Hi,we are planning to implement FIX for our internal purpose, would some one suggest initial guidelines to start,like which version should we start implementing and provide support for other version.
Also I would like to know has session level protocol changed much over various versions and where can I find the differences.

Please help me in our FIX implementation.

Thanks,
Srini