Firstly, what makes dart frogs challenging is that they are not very forgiving of mistakes. Unless you can maintain very high humidity, a stable temperature window of no lower than 65 F at night (about 18 degrees Celsius) and no higher than about 80 F during the day (about 27 degrees C), don't consider it. Also, you will need almost daily supplies of very small live food. For the vast majority of dart frog enthusiasts this means culturing your own fruit flies.
It's not that simple. There are many different kinds of dart frog and some require large areas to move around (some examples: Golden Poison Dart Frogs -
Phyllobates terribilis, also Dyeing Poison Dart Frogs -
Dendrobates tinctorious) , some are arboreal (most thumbnail species like the Imitating Dart Frog -
Dendrobates/Ranitomeya imitator), some are extremely territorial (Dyeing Poison Dart Frog is the best example - females literally kill each other), or a mixture of all of the above.
Probably the most forgiving to start off with would be one of the races/morphs of Dyeing Poison Dart Frog,
Dendrobates tinctorious. You could keep a pair in a 10g tank if it's landscaped correctly, but never more than 1 female unless you have a huge tank like a 75g. Since we
currently don't have any dart frog care articles, I recommend you read
Devin Edmond's excellent beginners guide.