Once you're able to get access, I believe you'll receive 15 credits per month for free. Each credit allows one generation, and each generation produces 4 images.
Rather than using up credits trying to learn how to formulate your prompts, I ran hundreds and uploaded them to https://generrated.com (I posted it a couple of weeks ago as a Show HN) — hopefully they might be useful as a starting point and save you some credits/money.