I have an older royce union mountain bike. Yes, I know its a POS but my late father gave it to me— so I really want to fix it and use it as my main bike (i really like the frame). The lower idler on the derailer is bad, it was shifting funny when it was rideable, the pedals are broke and I think it needs new brakes. I would also like a newer, more comfy seat.

Now, I know NOTHING about repairing bikes. I dont really want to pay a shop since that will be expensive. I want to outfit this bike with newer, better parts. But I have no idea where to start, what will fit, are all the sizes the same for bike parts? Totally lost here.