Poor credit score after the first 6 months

Hi everybody, so I have applied for credit card for 6 months now and I just got my score from TransUnion, it is 615. I applied on Discover as a student and got $500 limit. At first I thought if I used as much as I could to the limit the better because I thought they would increase my total limit, which led to today. Should I open another credit card since I am banking with navy federal and they offer a lot of good options for credit cards to increase my credit score? I need advice, thanks so much.
https://imgur.com/a/f6eOAcv
https://imgur.com/5CeJQCR