Randy's answer is correct, but we also use the present perfect to announce news (e.g., "There has been an earthquake in Mexico"). So if you are telling something to someone for the first time, or making an announcement, then the present perfect is appropriate. I think we would need to know the context of your sentence to know if the simple past vs. the present perfect were the best tense to use.

"I did this" means it was done at a specific time in the past. "I have done this" means it occurred at an indefinite time.