There are several ways to subscribe to subreddits.
If you are already on the subreddit you want to subscribe to, long-press on the subreddit name and tap “subscribe" on the pop out sidebar.
To add a specific subreddit, long-press on the word “front page” (or the current subreddit name) then tap into the field and type in the subreddit name and press the arrow to the right. Then once you are on the subreddit you can long-press on the name of the subreddit and select subscribe.
You can search for a subreddit using the magnifying glass icon and then check the checkbox when the subreddit is returned in search.
Finally, if youre reading a post that’s from a subreddit you wish to subscribe to, tap the “I” in the top toolbar to bring up the sidebar info and then tap “subscribe.”