I would start by taking off the top and lifting it out of the bottom casing as well. Take pictures as you go to remember where screws go - and have a bowl to put the screws in. There shouldn't be more than one or two big screws on the top.
Get to as much of the inner workings as you possibly can and get out any lint and dust you see. Sometimes a bit of kerosene on a q-tip is needed anywhere old grease has guilt up and is keeping things from moving smoothly when you turn the wheel by hand. (I just bought my youngest daughter a machine just like this one - and there was one place where I had to do this. The machine in the video now lives with my oldest. She is the one to thank for requesting the videos.)
Then get out some new (not vintage) sewing machine oil and just put a drop or two anywhere you see parts moving against each other. Keep turning the wheel slowly by hand until you think you've gotten all the parts then carefully put everything back together.
I usually get out a scrap to sew on and a paper towel to mop up in case I got carried away with the oil on the needle shaft. Give it a whir and see how things are going.
This is an excellent machine IF you keep it lint free and put the bobbin in right. DON'T use the cheap thread - it is made of short leftover fibers that shred themselves like crazy into your machine. The extra cost for good thread will save you a world of frustration with this machine (and all others as well.)
Oh - and needles dull too! Change your needle with each new project or whenever you hear that "pok, pok, pok" sound.