how do i set up my fuel gauge to work with a blazer tank?

[67cst]

i filled up my blazer tank a couple days ago and the gauge only read half, how do i get it to read full? i think a while ago i read in a post about reseting fuel gauges...im not sure how and id like to get this working asap, when i filled it up i made a 45 minute trip on the highway, the needle dropped 1/8th of tank when i got home, since then ive driven in town maybe 20 minutes and its down another quarter, so 3/8ths of a tank total, for some reason i dont think the needle is accurate, it shouldntve of dropped that soon i dont think, whats the deal with getting this gauge to work right? any help would be great, thanks guys

Derek