There is a current “Return to Duty” promotion that’s available to all players - a free “Jem’hadar Vanguard” character, which is normally a premium, upgraded version of a basic Jem’Hadar character. I keep forgetting to claim it for myself.

There’s also a starter pack on sale in the in-game store. It’s got some nice stuff in it, but I think you’d be fine without it if you don’t want to spend the money.