novice boarding the loft in need of help

Hi all,
I've finally got round to having a go at this but when I did I realised it wasn't quite as simple as I thought.
Here goes...
Next to the loft hatch three tongue and groove loft boards were already down which were flush with the edge of the hatch. Some other boards were up there already with nails in which had been pulled up.
The thing that I'm unsure about is that the 3 boards that were already laid down go completely over the joist on the side that I'm putting the other boards down on. Such that when I lay more boards down they don't rest on the joist at all on one end. I'm imagining stepping on one end and it becoming a seesaw. Has someone messed up with the boards that are down? Shouldn't they only go half way across the joist at either end of the board so that each board is resting on a joist in the middle and half a joist either end of each board?
I've heard that screws should be used instead of nails? Should I pull the nails out and use screws instead?
I've been looking for a guide somewhere on the net as I don't want to put the first few boards down and end up with all the others out of alignment and have to cut boards in half etc unneccessarily...

Many thanks

Pull up the boards and start again, use screws.

check the ceiling joist centres.

Boards are sized to suit 400mm or 600mm centres. If your house is older it may (like mine) have 450 centres - and your boards will not land on a joist.