Hi, yes the jam has started - it's fairly long (3 months) for a jam, there's still plenty of time to enter. (We don't expect people to spend an entire 3 months on their entry - the timeframe is there to ensure everyone who wants to take part gets the chance to do so!)
If you've never made a GBA game before, you can check out our getting started guide. Keep in mind that some programming skills are required (but there's always time to learn!)