Music is a language.
In my opinion for beginners of any age, the two most important subjects to study are:
1) music notation
2) basic chord vocabulary
In the beginning, practicing these two subjects makes the best use of your time. This is especially important for adult beginners who often have more limited time to practise.
1) MUSIC NOTATION
To understand music and most importantly to learn to think in music, the fundamental skill to work on is being able to read standard music notation. Not tablature. (All though I'm not totally against tab, because it does have it's uses.)
Regarding reading music notation, it is the path to building an understanding of music as a language, essentially leading one to interact more expressively and knowledgeably in many different musical circumstances.
Many people don't think it's necessary or think ''it's to hard"... well, it's true it's not easy.
Honestly, it's something I've never stopped working on. Having said this, you don't have to spend hours at it everyday to get reasonably good at it. 10-15 minutes a day early in a practice session is the best strategy and seems to work best for most people. There are several really good books for beginners that are easy to understand, are well structured with clear diagrams and illustrations, that make learning to read music interesting and enjoyable. Always remember it's a language.
2) BASIC CHORD VOCABULARY
I recommend learning basic chords at the same time as learning to read notation.
Studying diagrams, one memorizes various chord shapes to play basic chord progressions with a basic rhythm. As soon as I feel that a student is changing chords with a basic rhythmic flow I encourage them to learn a simple song and help make them aware of how to play along with the track. Most importantly I urge students to learn to play from the beginning of the song to end of the song. This is a very important habit to form. There are so many simple songs to choose from ( Beatles-Coldplay...etc) Often these days I get many students who have been playing for years and know only fragments of songs.
I help beginner students build a wide body of simpler songs to study, slowly increasing complexity. I find this a most successful strategy in helping beginners build a good chord vocabulary in a more permanent way, because one encounters the chords in a musical context therefore slowly cultivating "an ear" or a knowing of the expressive quality of each of the chords.
In closing the study of music notation and chords will help a beginner utilize their time wisely, and help them interact with others in more rich and meaningful ways.