I'm laying tile in my bathroom where there used to be linoleum. The floor butts to carpeting and I've been told I should use a metal strip between the two surfaces. I bought the strip and have cut it to fit but my question is how do I adhere it down? It looks like I could screw it down or possibly glue it. I don't know which would be better.
I'm very concerned that the tiles be straight (and I'm quite sure that the bathroom isnt).

Any help or suggestions would be appreciated!

It sounds like you bought a strip that doesn't have the 'tack' edge on the inside edge.

A carpet strip nails or screws down and is bent over the carpet,the back side only touches the tiles.

The threshold edge, which is what it sounds like you bought, will cover both edges, like a transition, and has holes in the center.It can be used for your purposes, just make sure the screws are not up and have no sharp edges to catch bare feet!

The tiling should end square with the door opening and the transition will look clean and proper, laying the tile from the center of the room, finding the right spot is in the layout section of the tiling mfgrs' paperwork(hopefully)

Good luck,

Ken

What you should have bought was a tile threshold and butted it to the carpet and laid the tile then, since you can't now you may try to use some liquid nails underneath the metal piece you have and weight it down till it drys, as for the squarness of it, if it looks like there may be something off then just eye ball it to see what looks more square,
If you don't want to glue it, you can use nails that come with it, or should have came with it, the threshold will need to be wide enough to lap of the tile a good 3/4 and then onto the carpet an 1" at least. Nail it through the carpet

There are many options. I prefer safetstrip.

A couple of tiles in my bathroom has peeled off. How do I fix it and what glue to use

First you need to post your own question on a separete post. As to your question your need to replace those pieces, pull them all the way up and remove any adhesive that is left behind, then make sure there is no dust or moisture on the subfloor and reset the new tile.
If you want to try and salvage the ones still there you may try use some floor adhesive called sticky stuff from Lowe's or another brand, don't put a lot on the subfloor but just enough to do the job, lay a book or something with some weight on top of the tile while it sets