We also have a great range of coloured tiles which you can use. If cutting your own tiles is a bit daunting for a first project, I can suggest using the pre-cut tiles similar to Mandala Art Mosaic Glass Tiles. For my first project, I used the pre-cut pieces and found it a breeze to create something I was proud of. I'm not sure if it is a snail or a nautilus, but I'll include an image below either way.
For my first mosaic, I simply cut some plywood board as a base and used the pre-cut pieces. I printed a template of the image I wanted to achieve (nautilus) then transferred it onto the board to give me a guide to follow. I added glue to the board and placed my tiles until my image was complete. I followed that by grouting the tiles and cleaning up the excess.
If you wish the board to be hung outside I would recommend marine plywood which you can paint to assist in preventing moisture damage. To build up a 3D effect on the fish, I would use Selleys 250g Clear Liquid Nails Construction Adhesive which is more viscous than the tile adhesive I mentioned. You can build the adhesive up in layers until you reach the desired body thickness. - MitchellMc
Don't be afraid to cut tile or glass you've got heaps of chances to try over and over again! I suggest using a tile and grout sealer to preserve the finish and it will so seal the colour of your grout. Looking forward to your future creations. I hope you have more luck posting next time. - redracer01