Peel and chop the onion into smallish pieces, chop the two cloves of garlic into tiny pieces. fry them in a small amount of oil (preferably olive oil) for a few minutes on medium heat, stirring constantly until the onion has started to go clear.
Add the mince to the sauce pan and cook until it has all changed colour and started to go "fluffy".
At this point you can drain off all the juices that have come out of the meat if you wish (i never do as it adds to the flavour).
Start boiling a saucepan of water whilst the sauce cooks, once the sauce is starting to look reasonably thick you can drop the spaghetti into the boiling water and it should just curl round the saucepan.
The sauce may look a bit watery and not very promising at first but once left to bubble away for half an hour or so it will have thickened up, you can cook the sauce for anywhere from half an hour to two hours (if you have the time), the longer you take the better it will taste at the end.