Start with getting a repair manual. Clymer's or Haynes, both ok. Both have slight mistakes. I like the factory books. Free download at biker.net.
Can be found on Ebay.
Start with an oil change, pull the oil filters, there are two. One in the right side engine cover behind the small round cover. The other is on the sump plate on the bottom of the engine.
Check both for debris. Clean and reinstall. The sump filter is often torn. Lots of us patch the tear with JB Weld.
Pull the battery, get a motorcycle specific charger, charge the battery. Take it to a place that sells batteries and get it load tested. If it passes the load test, you can use it for awhile. It probably needs replacing.
Start at one end of the bike and work toward the other, checking each any every electrical connection for clean and tight.
There is a sequence of things to do during a tune up. Do these things in order.
Adjust the cam chain tensioner
Adjust the valves
On the 70-79 points bikes check the points gap and timing.
On the 80 up they used an electronic ignition, so no points.
This ignition is a TCI or Transistor Controlled Ignition.
I would also suggest you read this,
www.amckayltd.com/carbguide.pdf This will explain the carbs very well. How to tear down, clean, inspect, adjust. You may need to do this. Cleaning the tank and petcock won't hurt either.
Now with the electrical system clean and tight, fresh oil, clean carbs and tank, good battery it just may start and run.
If so enjoy.
If not just ask and we can help.
This and reading through the XS650 TECH section, there are links to threads about most every question you might have in there.
You didn't mention the year or model of your bike. A lot of us put this info in our signature, accessible in your profile.
Good luck and have fun.
Leo