Hi Vasileia, this is a quite big subject and, like many other times, there's not really just one answer.
For sure, if you want to jump from the normal coffee, (the "commodity" coffee you can usually buy cheap in the supermarket) to something better, I can suggest you to start to look online for some specialty coffee roaster (or even a simple specialty coffee shop). Here you can definitely start to look for something good! Of course I'm not saying that all the specialty coffee are good, at least in term of the final result in the cup, but at least choosing a coffee that was graded specialty (80+ on a scale up to 100) you will for sure find something potentially really good.
Specialty coffee (almost) always means Arabica coffee, and that's good.
Then check the production date and the "best before" date. If it's roasted in the last two weeks (up to one-two months) it's going to be probably really nice.
Regarding light-medium-dark roats, give it a try... you'll soon discover what's best fo you (I'm totally for the light roast, but it's really just a preference).
Same for the country of origins (at the beginning my suggestion is simple: check the labels for aromas and flavors and just pick what seems appealing to you!)
Blend Vs. Single Origin: I go always for single origin (because I love to discover new flavors, and because my favorite brewing method is always filter), but a nice blend can be perfect if you want to have a nice, chocolaty cappuccino.
It seems a lot, I know, but step by step you'll start to learn more, and you'll soon discover how amazing is this coffee world!