Compression of a text that contains only the 5 first letters of the alphabet? So the question is: "Imagine the compression of a text written only with the first 5 characters of the alphabet (a, b, c, d, e). Knowing that their apparition's frequency is 1)e, 2)a, 3)c, 4)d, 5)b, code the letters on the minimum bits possible."

so here's how I did start: 

We have 5 characters, so 3bits will be enough for the characters (2^3 =8). We can put: e=001, a=010, c=100, d=011, b=110 But I don't see how to use their frequency (we don't have the value of it, they just gave a "scale" of their frequency)
If you look at the algorithm for creating Huffman trees/codes, you will see that while it uses frequencies, any weight works. You can always rescale (by dividing by the sum of all weights), if you wish.

If your problem does not contain any weights but only a relative order, you can not build a Huffman tree; you don't know where in the order the inner nodes you create go. Ask your teacher.