You ask questions. You don't start the work until you understand what the work is, and you find that out by asking questions. If you are given the assignment in person, don't end the meeting until you have asked enough questions. If you are given the assignment in email, reply with your questions or with an email asking for a phone call or meeting to ask questions.
Yes, it's frustrating to have to interview requirements out of people. We have a saying in my company: some clients really need consultants. If they knew precisely what they wanted we would just be typing, right?
So, in your case "I want an excel file showing the comparison between two different softwares" you might ask
- have you chosen the two software packages or is my first step to research and find what packages to consider?
- is this spreadsheet going to be one column for each software package, and then rows with the price, features, and so on? [I would expect a yes here, but it doesn't hurt to ask.]
- have you made a list already of the features you care about, or should I research what the features are?
- when do you need this by?
- if I can't find out some of the information, should I give you the partial spreadsheet, or let you know it will be late?
As time goes by, you may need to ask less questions because you may realize that your boss wants you to figure out how to do these things. Or you may ask less questions because your boss realizes that it's important to give you more detailed instructions. But in the short term, the only fix for incomplete requirements is to ask short, crisp questions (not "I don't understand, can you give me more details" or "I need more requirements than that") until you have the requirements you need.