new boy needs advice about light cycles!

thanks guys for the advice from my last post least i'm know i'm going in the right direction but could anyone explain a bit deeper in wat i will get out of keeping my plants on 18hrs??? also im struggling to keep the temp down in there, ive got a fan suckin out the hot air its not connected to the filter yet as it does not smell and i have a little fan just circling the air but my temp on a really hot day can touch 90 but average is about 86 cause its warm at the mo! is this to high i'm worried??? also anyone got any answers to my yellow leave problem??. and one more thing lol! ive just brought some bigbud seeds thought i'd have a ago at raising some as i actually dont know wat strain mine are!!! but now i think bigbuds a wrong move just seen some reviews not very good! im looking to get something with half decent yeild and with a max height of around 1.5 m and suggestions in wat i should try?? want something strongish thanks guys!