ebay for beginners...

Now that I'm connected to the net at home I can finally use ebay. I want to sell loads of junk and other bits and pieces to put towards my Hawaii fund this summer.
So, im totally ignorant when it comes to stuff like this. How exactly does ebay work? How do you actually sell stuff and know that the person buying will pay you the money? I'm assuming that ebay makes a profit on your transaction too... do they take a percentage or something and how does that work? When you post stuff how do you know how much it's gonna cost? do you add postal charges before selling it (how?) or do you just accept that you'll lose a few quid for stamps and stuff?
Also... I was thinking of selling my pedal bike (which I hardly use anymore
) but you can't really send big items like that through the post - would you expect a complete stranger to come round your house???
Any help appreciated!
So, im totally ignorant when it comes to stuff like this. How exactly does ebay work? How do you actually sell stuff and know that the person buying will pay you the money? I'm assuming that ebay makes a profit on your transaction too... do they take a percentage or something and how does that work? When you post stuff how do you know how much it's gonna cost? do you add postal charges before selling it (how?) or do you just accept that you'll lose a few quid for stamps and stuff?
Also... I was thinking of selling my pedal bike (which I hardly use anymore

Any help appreciated!