Giorgios, it is commendable that you are interested in this and ask such questions. But you are still thinking as a classical scientist. As Peter says, you do not really need the concept of a particle in qft. Fields are enough. If you really want to understand this better, alas you will have to study qft. The concept of virtual particles comes from an extremely ingenious way that Feynman found to represent a perturbative expansion graphically.