Yes, it is allowed to upload Ukulele songs in Germany. But, in any case, it is advised to respect the local copyrights laws in the country.
You can upload videos directly from your phone or laptop. If you are using laptop, you have to click on this icon to start uploading or posting the video:
If you are using your phone, you have to launch your YouTube app and then click on camera icon:
On next screen, you have to follow the instructions to start uploading the video. Make sure, you give it an attractive title and description to reach out to maximum viewers.
Please revert in case of any query!