There’s no such thing as “good enough” for tournaments. If you’re talking about accomplishing specific goals in a tournament, such as placing top 8 in your locals, or something, it depends primarily on your local competition.
If you mean “how good do you need to be before you join the first tournament”, it really depends on you. There’s no requirement for joining a tournament, other than an entry fee, so whenever you feel comfortable in your ability to play in a competitive setting, is when you’re ready for a tournament.
But tournaments are a very different animal than playing casuals, the computer, trial mode, or training mode. Tournaments introduce a degree of stress and a pressure to perform, and require that you be able to adapt to your opponent and be willing to change your strategy on the fly, which is a very difficult thing to do.