I tried to cook croissants following a recipe I found on internet, but although I used yeast they didn't grow at all once in the oven. Is there any secret to prepare the dough?
The answer to this question is essentially the same as the question "how do you make croissants".
If you are making croissants du boulanger, the dough has yeast in it; if you are making croissants du pattissier, the dough does not have yeast in it.
The fact that croissants can be made without yeast shows that the yeast itself is not essential to the oven spring or rise. The yeast in croissants du boulanger is primarily for flavor development.
The actual rise in croissants, as in puff pastry, comes from the laminated nature of the dough. The dough consists of a series of layers of flour-water dough, interspersed with butter. When baked, the steam released from the dough causes the pastry to expand; the fat layers prevent the layers from staying glued together long enough for them to set separately before the butter melts.
In summary, a layer of solid butter is encased in solid dough. This is then folded over itself. It is then rolled thinner, and the folding repeated several times. Practically, during the several folds, the dough must be allowed to relax and chill.
Assuming you tried to follow this method, the likely problems that prevented the rise or flakiness from your dough are: