In this thesis, properties of carbon based nanowires are studied by ab-initio calculations. The aim is to gain a thorough understanding of the electronic, spintronic, transport properties in nanowires and how they are affected by different geometric formations, defects and adatom adsorptions. To this end the non-equilibrium Green's function formalism with first principles pseudopotential density functional theory calculations have been used to describe spin-polarized systems. Firstly, different geometric formations of Cobalt-Benzene nanowires are investigated. Systems with ferromagnetic ordering are calculated as half-metallic while systems with antiferromagnetic ordering behave as metallic. Also the results of the spin polarized current calculations indicate that one of the spin components of current is dominant for the antiferromagnetic systems while both spin components of current are dominant in different bias windows of a specific total applied bias. As second case, alkali atom termination of the zigzag graphene nanoribbons (ZGNR) are studied. In particular, using sodium atoms for the saturation of ZGNR edges at half the concentration of edge-carbon atoms make it a one dimensional, perfect semimetal, where the valance and conduction bands meet at only a single, Dirac-like point. Unlike pristine graphene, the Dirac-"cones" of Na-ZGNR is not symmetric with respect to wave vector, but rather it is tilted. Finally, adsorption up to the graphenic sheets with periodic 5-8 defects is studied. Especially, electronic structure of the V adsorption into 5-8 defects induced graphenic sheets are calculated as half-metallic while formation of linear bands crossing at the Fermi level which form a tilted Dirac cone.