A vintage motorbike, like any other bike, will require periodic maintenance and occasional repairs to run it’s best. Vintage motorbikes are not nearly as complicated as modern machines and even an inexperienced mechanic will be able to do many maintenance tasks.
If you are inexperienced, start out cleaning and inspecting your bike. Become familiar with the different parts and what they do. When you do start bigger jobs, be sure you understand what will be required before you start. Take plenty of photos before you take things apart so you will have a reference when it comes time to put things back together.
I guess in a way, this is the 6 million dollar question. For me, the starting point is always where it will get the bike to a running condition fastest. I use this method simply because if I can get the bike moving under it’s own power, I can assess other areas where it needs work.