I am laying porcelain tile on bathroom walls in shower, should I put metal mesh against wall to hold tile up better.

Skip the metal mesh and use something like Wonderboard, concrete board or Hardiebacker board

Bob is correct about using a cement board for the walls of the shower. If there is drywall I would remove it and replace it with cement board.

As the others have stated, use 1/2" cement board on the walls with either a moisture barrier of 15lb roofing felt or 4 mil plastic sheeting lining the studs first.

You could skip the felt or poly and use a topical waterproofer over the cement board first, instead.

No lath will make the tile stay better...floor or wall. If you are doing a mud job, that is 1.25" of mud over tar paper and lath on the floor, then the lath is there to reenforce the muc bed just like rebar would be used in concrete.

Lath on the walls is needed if you are floating the walls

Lath for self leveling cements poured a minimum of 3/8" over plywood

Lath for reinforcing the preslope bed of a shower or for forming the shower curb.

Lath for anything else, such as skim coating it with thinset after fastening it to the floor... well that's an indication that the installer you are about to hire should be thrown out of the home and his business card burned so that no one else might accidentally get that person's number from a business card you once possessed. As a matter of fact, you should take a sharpie marker with you and everywhere you go, ask to borrow their phone book and black out the listing with the sharpie marker. Did I happen to say it's a garbage hackish install method? Did you know that has never ever passed any independent industry testing with a very early on failure rate?

And when you set this tile in the shower, you should never be using mastic or premixed thinset from a bucket. Any bagged modified thinset will do. Specialty nonsag thinsets will make it even easier, such as laticrete 255, tec 3n1 or custom's megalite.