Use the Civopedia to check on the prerequisite requirements for each tech you want. Only the eight starting techs do not require two other techs before you can research them, and there is the "hiding" of 1/3 of the techs each turn as well to contend with. Fusion requires NucPower and Superconductor, which in turn require NucFission and Electronics plus Plastics ans Laser. There is a downloadable tech chart in the download section if that would help as well (it might also be a PDF on your install disk).