Believe it or not, nontraditional students have to supply their high school transcripts when applying for admission to college--even if you graduated high school thirty years ago or more. For some nontraditional students, this is not easy to get. When my husband decided to attend some college classes, the school he where had graduated had closed for a period of ten years, due to low enrollment.
Fortunately, one of the administrative assistants of the school district was able to locate the transcript by going through boxes of old records that had been in a closet for many, many years. This lady, bless her, took the time to find his transcript. If she hadn't, it is possible he would not have been admitted without taking the GED (which was mentioned at one point by a college official).
As soon as you make the decision to be a student again, contact the high school counsellor where you graduated. Give them a long lead time to find the transcript, especially if you graduated before computers and even more time if you graduated before microfiche became popular for keeping records. This will make it more likely the transcript will arrive within the allotted time period. Even if you don't know where you want to attend college classes, most high school administrations would appreciate a heads up that you are planning to go and will be requesting the transcript soon.
As for the transcript contents, don't worry too much if your high school GPA was less than stellar. You just need it to show that you completed high school. Most colleges will require nontraditional students to take the ACT, SAT, or a some kind of placement test to give them an idea where to place you as far as Math and English courses go. They want to make sure they don't put you in an Algebra class if you need to brush up on basic mathematics and pre-algebra, etc.
Going to school and being a student again is stressful enough without worrying about the highschool transcript Just be sure you request it early, then spend your worry time prepping for the ACT or SAT!