You need to leave a small gap in the corners and caulk it. Thinset probably won't stick to the caulk at the tub. WIth a small tile, you want it to be over 1/2 supported well, you can put thinset there, but leave a gap so you can caulk under it at the tub junction. Whether you run the short wall or the longer wall under depends on the layout, then bring the other wall's tile to it. You need a clean line, so see which wall is straighter and will look better. Use the appropriate thinset (often a highly modified thinset) specified by the tile manufacturer. Usually, you'll want to use white which costs a little more, but you'll get less of a color change. If you don't get full coverage, and the tile is not opaque, you'll see the thinset underneath, so be careful. You need to bed it in the thinset so you can no longer see the notches in the thinset. Sometimes, it is a good idea to burn in a layer of thinset on the backs of the tile first to ensure a good consistent layer, but then you end up with a major pain getting it out of the joints. Glass tile is not something I'd want to do as a first tiling job, but take your time and do good cleanup while you go and you'll be okay. DO NOT let the thinset skin over before setting the tile. It will take you awhile, so don't spread too much at one time.