While moving to SharePoint cloud version from SharePoint Server,the major issue faced is to migrate the contents to SharePoint Online.
There are lot many tools available in the market which can do this job for you, but some are paid and some are not working as per our requirement.
Finally keeping this all things in mind, Microsoft came up with its own free tool which will help you to migrate the contents from SharePoint On Premises to SharePoint Online.
You can get the information about tool here
And for instruction you can refer
Note: Currently this tool works only for SharePoint 2013 to SharePoint Online only. It will not work for any previous versions.