Start out in small surf in plenty of wind... The same wind as you normally would ride with a twintip...
You hear about all this business of getting going earlier/lighter/sooner on a surfboard - but in reality it takes 2-3 months of riding a surfboard to get the hang of it - and only then will the light wind benefits open up to you.
Start out on your 6"5 fish - but in my experience the fish boards don't work too well behind a kite - the rails are too soft and wide up on the nose which makes for a funny feeling board. The foamies don't work due to not having real fins...
Try it out on your fish first - see if you like it :) Then go grab a cheapie shortboard from your local surf shop to really get into it
