Once you've found a forum that looks good, has lots of members, and knows what's up, post! Remember to give ALL the information you possibly can. Pictures are usually a must.
Tank size, water change frequency, number of animals in the tank/ their species, how long you've had the betta, how often/much the fish is fed, (if you have it) water parameters like pH hardness ammonia nitrate and nitrite levels, any decor that is not 100% made for fish, if the tank has a filter, if it has a heater (it should!), what temperature, special notes/symptoms, etc.
I (and BSp) don't affiliate with any fish-specific forum, but will likely make a guide on how to best use a forum and how to get the answers you need fast and effectively very soon :) !
If you've received a reply from BSp already, I will continue to reply to your questions until everything's all set, don't worry. We'll continue what we started!