Will I harm my reputation in a company by rejecting offers?

I live in a small town and I wouldn't say that there are a lot of jobs available. So I'm applying my CV's to a companies big and small, wether it's internships or full-time or courses that companies are offering. I'm usually accepted but I decline opportunities because something else is came out and recently I decided to study something else. I've yet to establish myself there and I don't know what interests me a lot. Usually job opportunities are for at least 6 months which is a big time to do something that I'm not sure of doing. But I got very anxious that my actions leave bad influence on HRs. Will I be accepted to that company again if I decline offers? I have accepted recently to procurement course and I'm not sure should I take it or not. But this company is very well-known and I don't want to harm my reputation there or anywhere. I would be more aware after applying my CV's in the future. Did I ruin my reputation? How can I improve it?