In life, and in this forum, we need to ask specific questions to get specific answers because when we ask vague questions we get... even more questions as people trying to understand what you actually mean.
If I tell you, "Hey, I can't drive, please help me" can you give me a one-answer solution? You have to ask, why? what is the problem? Car won't start? you run out of gas? are you drunk? do you even know how to drive? etc and each problem has different solutions.
The same goes for your question? Is it youtube? is it Netflix? have you tried and failed? if you failed with what error? Have you never done it before and don't know how? What DRM is used? Are you sure it's DRMed? Do you appear in the right geolocation?
So to make it simple. Google "How to download videos from YYYY service" Read the guides and follow them. If they ask for software, come here and check if we have it.
Nothing else to add so the thread will close.