Is a 20.15% APR rate too high for my first credit card?

That's really high but since it's your first credit card it's not unheard of at all. Be sure to always pay off the full balance each month. Since your limit is only $600 that shouldn't be too hard. Be very careful with credit cards. You're trying to build your credit. Don't overdo it. (Like I did... sigh) Good luck!