Chose a Video editing software ... ( Windows movie maker perhaps - discontinued )
Once you have video editing software ...
You open the files you want to make into a movie ...
In the editor , you can join / cut / do special effects / audio ( bla bla ) ...
Most importantly , you can chose a Codec and quality to save to .. There by ending up with a smaller file size ..
Then you need to join youtube ( create an account ) ...
When you have an account you can then upload to that account and make the movie / video public or private ..
Check out youtube for plenty of tutorials ...