Establishing and building credit with a mediocre-poor Credit Score

Hello! As the title says, I’m looking to establish and build some credit and ultimately boost my currently low (620 or so) credit score. I’m wondering what my best first step would be, should I sign up for one of these preapproved $2000 credit lines I get sent every so often? Should I be seeking out a certain first card (or two) to get going? I’m afraid of applying for a card and getting rejected. I work in the hospitality industry (restaurant server.) Thanks!!

**to clarify, I currently don’t have an open credit line, I just use my debit card and cash for everything***