I want to tile a picnic table top the top has rotted away what materi

Is the surface level and even? If not you will need to attach a board to the top of the table for stability, then set the tiles on that, then grout, then seal.

I agree with Naomie - If it is not done right it will not last (Make sure your Grout is waterproof)

I also agree but if this is left outside all the time, I think I would use cement board of top and then the tile. I love your idea of tiling your table. Post pictures when you are done and let us know what you did.

you will need to create a base on which to lay the tile first...any wood will do. Then apply the tile adhesive with a serated trowel, butter the tile and lay it down. Once the tile has dried, grout in between the tiles.

I would create a top of cedar (doesn't rot) and then add a layer of cement board over the top before adding the tiles.