Like many a self taught guitarists I got a nasty shock when I first played in a band. I sped up, slowed down and even just stopped at random.

Learn to play in time from the outset. I have an old fashioned wooden mechanical Metronome which lives in the lounge so it is handy to use. It is also a thing of beauty. A cheap electronic one is in my gig bag. When playing bass, l use the drum machine built into my effects pedal. Indeed that and the headphone output were the reasons for buying the pedal in the first place!

A basic tuner is cheap, but will only do the 6 notes of a guitar in standard tuning. E A D G B E. This is convenient, and it does help you to remember what standard tuning is. However the time will come when you want to use a different tuning or tune a different instrument (your kids violin, a Stylophone!). A chromatic tuner will recognise and let you tune to any note in the chromatic scale, hence the name.

Adjustable reference frequency is great if you find yourself having to play with an instrument which is not, and cannot be tuned to concert pitch. I had this situation in the swing band. The Piano was flat. I would normally have tuned directly to sound of the piano by ear, but in a rehearsal room full of brass instruments this was a none starter. Even if it was possible with that racket going on, I didn't have time. I solved the problem by getting there early, just the once. I quickly establishing that, on the piano, middle A sounded at just 435 Hz. Concert pitch is 440 Hz. I could now tune up silently just by setting the reference frequency on my tuner to 435.