Graphene - a two-dimensional honeycomb lattice of sp2-bonded carbon atoms - has been attracting a great deal of research interest since its first experimental realization in 2004, due to its various novel properties and its potential for applications in futuristic nanodevices. Many of its applications require tuning of its properties, which can be achieved by varying the number of layers or/and by doping. There are several ways to dope graphene: (i) electrochemically gated doping, (ii) molecular charge-transfer doping, and (iii) substitutional doping by atoms like Boron or Nitrogen. Moreover, for graphene, a zero band gap semiconductor in its pristine form, to become a versatile electronic device material it is mandatory to find means to open up a band gap and tune the size of the band gap. Motivated by these considerations, we here present a systematic and thorough study of the structural, electronic and vibrational properties of graphene and their dependence on the number of layers, and on doping achieved electrochemically, molecularly and substitutionally, using first principles density functional theory (DFT).