You should be able to just by clicking the button in the sidebar.
On mobile, you might have to unhide the sidebar first.
Or go to lemm.ee/communities?listingType=All and search for communities that interest you. There should be a subscribe button next to each one.

Other that that, it depends on the app you use to access lemmy.
In my opinion, just the web interface in a browser is good, on mobile too.