Good job! With 6 months of good history you should qualify for an Amex charge card, the Premier Rewards Gold is a highly recommended card and easy to get. Have you checked the prequal sites of Capital One and Amex?
Gardening until September 2025
You're off to a great start! 
With 6 months history, you can probably get an Amex (which one were you looking at?). Chase might want closer to a year of history (I recommend waiting because you don't want to get stuck with a $500 Freedom that will never grow), not sure about Citi.
Quicksilver $10,000 | Better Balance Rewards $2000 | Sallie Mae $3500 | Freedom $3500
First off congrats on taking the first step to building your credit!
If you are generating a FICO score now I'd say go for an AMEX. Any one you'd like. I was approved for my First AMEX (BCE) back in 2014 with 6 months of history and $12,000 income just like you. The starting limit was $8,000. Amex is very picky about clean files, but they seem to love newcomers to credit as long as you can generate those magical three digit numbers.
Chase I would definitely hold off until you've got about 1 year of history under your belt and 6 months of history under your AMEX. They seem to like that 1 year spot and people that have more than 1 credit card Tradeline reporting, so that AMEX will unlock that Chase door for you.
Same deal with Citibank, although they might be willing to flex a little bit if you manage your credit cards with excellence. I applied for the Citi Double Cash with 11 months of history under my belt. I was initially sent a declined letter in the mail stating not enough revolving history. I recon'd that with the Executive Office and was approved for $500 to start, but it has grown exponentially in the last year (see signature below). If you want an easier approval without having to recon and risk taking a $500 starting limit, then I would give it a bit more time.
Good luck!
