Well, you don't have any revolving credit, which is a big driver of FICO scores.
Those new to credit (just hitting 6 months) generally seem to have scores around 680 or so. I don't know if yours would be that high, since you only have installment.
In the long run, it would be wise to have a couple of CC's, but there's no hurry. The longer you wait, the better a deal you're apt to get. I would certainly wait until you have six months of history on your auto loan reporting before getting a first card, and then I'd recommend waiting a second six months (year total) for a second card.
That will give you time to do some research and figure out which card or cards might make the most sense for your usage. Think "rewards." 
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