You can use automatic dishwasher liquid. It's made to cut through grime as well as water spots and calcium build-up. You might want to dilute it enough with water (as it is usually very thick) so you can use it in a spray bottle. If not, just pour some on a sponge and wipe it on. You'll need to let it sit a while, but it will remove the lime spots wonderfully.
After you clean, get a squeegie and use it every time a shower is taken.
One other tip. After you clean the glass (even the walls), apply a coat of automobile paste wax before using the shower. Buff out the wax clear. Then use a squeegie and you'll always have a very, very clean shower.
I did this more than a year ago on my home shower, and I have not had to clean the glass since. The squeegie takes care of it.
But for your initial cleaning, Automatic Dishwashing Liquid (any brand) works very well.