Getting your bike back in shape is rather daunting task. I would probably leave the whole process to a professional rather than making a mess by myself. Bike maintenance can be complex, and it’s better to let the professionals handle it. Still it is always better to have a knowledge of some of the critical parts of the bike and their working. Checking for corrosion is most important. Chain, brake pads, tyres, cables, electricals are just a few things which can be serviced with some help. I do not prefer to buy spares online but rather would get it from authorised dealer. But as I said, usually I don’t take the trouble and let the professionals handle it.