Language Decidability
A language $L$ is decidable if there is an algorithm that halts on every input, says Yes if the input belongs to $L$, and says No if the input doesn't belong to $L$.

The language of all binary representations of prime numbers is decidable since there is an algorithm that decides whether an integer is prime. You can probably code such an algorithm yourself in your favorite programming language. There are a few added complications - you need to parse the input, and you need to handle arbitrarily large integers - but intuitively these are not hard to address.

Similarly, the language consisting of only the empty string is decidable, since an algorithm can examine the string and check whether it is empty or not.

The real challenge is proving that some problem ins't decidable. The main techniques are are diagonalization and reduction. Roughly speaking, proofs by diagonalization construct an input that every potential algorithm fails to address, using some kind of self-reference. A good example is the halting problem, much information on which is available online. Proofs by reduction show that if you could decide your problem X then you would be able to decide some other problem Y which is already known to be undecidable.