how can i work any video format on my nokia 6620??

Please define "work" and "any video format"?

If "work" = "play", then the built-in Real Player can play some video formats (not any): 3GPP, Real Video 7, Real Video 8.

For more formats, install an app like SmartVideo (you can buy it from the shop http://www.allaboutsymbian.com/shop/) or some other such app (PacketVideo, Oplayo, DVD to Mobile, etc.).

Use also the site's search function for finding existing discussions: http://www.allaboutsymbian.com/forum/search.php

Note: "any video format" can never be equal to "every video format that exists in the world". 😉