Conversion Software question

I have an idea I want to implement and test the youtube waters. I am years late to this game and many of you senior members have tapped all the cash there is to make but id still like to take a stab at it

I need to take videos from other tube sites and be able to edit them. I have a niche id like to try out but need to do this first as my target videos are on a different site.

Any insight as to what I can download to do this?

- download
- convert if necessary to other formats
- edit the video

those are my requirements, thanks in advance