How to sand and varish an old wooden floor

[spotit3]

I have a wooden floor in my house made from reclaimed floorboards. It has originally been varnished and is also very dark in places. A friend suggested hiring an electric sander, sanding it down and revarnishing it but being a female who has never before had to do anything around the house would appreciate some advice on how exactly to do it. I don't want to make it worse than it already is!!

[heathfield]

I'd very strongly advise getting a professional firm to do the sanding. They wouldn't take long to do it, and you'd get a proper finish. The only thing you must do is to go carefully over the whole floor beforehand and make sure that there's not one single nail or screw left that would interfere with the sander.
As to varnish, I'd recommend Sandolin. You'll have seen their varnishes on shop floors. It stands up to wear and tear from foot traffic much better than most other makes.

[spotit3]

Thanks to both Heathfield and Zacsmaster. I have taken your advice and have a professional coming to look at it and give me a price next week so hopefully he will do a good job on it.