Hun, I know this is going to sound like a total cop-out, but the answer to your question really is: "when you feel you are ready".
Some people feel ready the very instant they realise they're trans.
Others take weeks, months, years or even decades.
Maybe explain to them that you're not completely sure about it but want to try it out. If you try it with a small group to start with, you can always say later on "actually you know what, I don't feel like this is really me" if you change your mind.
That would be my advice to you hun. Hope I've helped
<333