I was looking for free stock video and came across this useful listing of stock video sites.
To combine stock video you need a video editor and I found this listing on techradar useful.
I tried Lightwork s and it was really hard to use. An easier choice is to use windows movie maker which is free, but has been discontinued. You can still find it and download it for example here.
Some professionals recommend using Adobe Premiere Pro, but that requires a paid license.