If you want to get ahead early, build GL and NC in the capital by turn 65 (doable with a good balance of hammers and food).
However, you can totally forget about GL and build NC by turn 150 and still be "in the mix". If you don't have a NC by this point, it is still worth building later on but you won't get as much effect from it. Not building it at all will usually mean you will fall behind in tech unless you have other science buffers in place (Observatory, science-heavy religion, Patronage+Allies, early Rationalism opener, etc.). Generally on Emperor, you don't have to beeline the science techs to get an eventual tech lead. I usually beeline for the situational wonders which would make a bigger difference to me in the long run (such as Petra, Great Mosque of Djenne, Forbidden Palace, etc.).
Emperor is a good training ground in my opinion. It will allow you to try different tactics out but still be able to recuperate if they go wrong. For your first few games, try Science heavy starts, then build up from there.