Sounds like you have an old bombora tri fin (tri fins are back in vogue now). Ditch the old gear. You can get some used gear that will be lighter, easier to use and a huge jump in performance over what you have now.
There is plenty of gear available on the 'buy and sell' part of this forum, however if you are not familiar with the new gear you are best going to a shop for advice.
Go and see Sam or Chris at WindSurfnSnow at Longreef. They will give you good advice for an initial set up and once you get familiar you can then get more used bits from this site.
wssboards.com.au/