The modern software for Finite Element Method calculations is a powerful tool when making strength and stiffness calculations. However, this is only the case when FEM is applied correctly. This work contains all the information necessary for a thorough understanding of the FEM and its correct application: - how are structures to be modelled into building blocks that can be used for calculations in an FEM program? - how are FEM programs organised internally? Part one of this work explains how the FEM works using two-dimensional truss and beam elements. Part two will discuss the three-dimensional structures such as plate elements. Attention will also be given to the dynamic behaviour of structures.