How do I get Helvetica font in Photoshop?

What's the Alternative called then?

There are so many free font sites out there where you can download fonts for free.....mac or pc. If you are using a Mac it might be wise to use a font manager. Then you can turn your fonts on and off as needed.

Further to what you have already been told, you might like to check the Wikipedia entry for Helvetica

https://en.wikipedia.org/wiki/Helvetica

It has pretty much become a font class nowadays, but there are sans serif fonts in Photoshop that will do the trick.  I was excited when TypeKit was introduced, but later became disillusioned when I realised how many useful fonts were no longer available with CC.  Bickham Script Pro, for instance, is a font used by one of my wine producing clients, so I was not impressed when that disappeared after CS6.  Getting back to your question, Photoshop used to have a range of Swiss type faces that could have been used for the sort of banner headline applications Helvetica is associated with, but that has also gone.  I went as far as emailing the TypeKit team about it a while back, and was fobbed off with a meaningless reply.

If you have a pre CC version of CS6 installed, then you will have the old fonts.  In my case I had CS6 installed on my laptop, so was able to copy and transfer the font files to my desktop.  It still annoyed me deeply that I had to go to that much trouble!

It needs to be installed on your system.