IMO, the whole thing started at issue 7 of GLC, and 6 issues after OYL on GL. There was small build up to it, like GUy being framed, Mogo being infected and such. The actual Sinestro Corps books start at Green Lantern 21 and GLC 14. Then theres the one shot, if you can find it.
From wiki;
The main story consists of ten parts plus an epilogue, running through the GL and GLC books. Part One was released in June as the Green Lantern Sinestro Corps Special #1. Parts Two through Ten, which will be released between August and December, alternate between Green Lantern #21-25 and Green Lantern Corps #14-18, with an epilogue in Green Lantern #26.[2] Blue Beetle #20 and Green Lantern Corps #18-19 are also connected to the story.[2] December will also see the release of Green Lantern/Sinestro Corps Secret Files #1, further explaining the back story of both groups.
Personally, If you have the money, buy the whole run of GLC, and all the post OYL GL books. The first arc of GL OYL was alright, but since then it's been good. If you just want the Sinestro Corps story itself in GLC and GL, pick up from issues 14 and 21, respectivly, as well as the oneshot.