I´m at a turning point .. to change my OS from W7 to Ubuntu.
The thing is: Do I need to make a backup of my files?
Or, can I just install the new OS and my files will ALL be quite safe?
YES. All your files will be overwritten, by the new installation of Ubuntu.
Here are a few checks that you should seek:
In case both the answers to the above questions are yes, take a backup.