The easiest way to center a floor, is to measure and find out the centre on each of the sides, and then making a line between the opposite marks, but doing that without further consideration can result in very small sliver cuts.
For example, say you have a regular, fairly square floor, where side A is 3m, and side B is 2m, and you want to fix tiles which are 30x30cm in a regular grid pattern with 3mm grout lines.
Side A = 300cm / 30,3cm = 9,9 tiles
Side B = 200cm / 30,3cm = 6,6 tiles
If you just center the surface, Side A will have about 9/10s of a tile along the edges on both sides, while side B will have 1/3 cuts along the edges on both sides. 9/10s is as close to optimal as you get with those tiles and that grout line width. 1/3 is not optimal. To correct that, you'll have to offset the guideline half the width of a tile. That'll get you 8/10s of a tile along the edges on both sides of side B.
Here's the short of it:
If the last whole number of the number of tiles you can fit into a row is UNEVEN, you can just go ahead and mark the centre, because the cuts along the edges of the surface will always be atleast the width of half a tile, or more.
If the last whole number of the number of tiles you can fit into a row is EVEN, you need to offset the centre by half a tile, because otherwise the cuts along the edges of the surface will always be half a tile or less.
Maths. I think I'll write a proper guide on this tonight.
Anyways. The rest as grumpy says. You don't really want to start in the middle of the room if you can avoid it. dry lay a couple of tiles along you guide lines and mark a suitable point to start, and make a 90 degree line with your marking as a starting point. I'll think of an easier way to explain it later.
Good luck.