Electronic guitar tuners are amazingly cheap these days. You can get one for about £15 to £20. There is nothing worse when first starting to learn guitar than picking up an out of tune instrument that sounds like you are strangling a cat even when you are playing the correct notes.
Once you have chosen your guitar tuner (you will need one with a built in micrphone if you are tuning your acoustic guitar). Plug your guitar in and put the guitar on your knee in the playing position or on your guitar strap in the playing position.
Guitars should always be tuned in the playing position. If you try to tune it while it is lying on the floor or flat on its back on your knee it will sound out of tune as soon as you put it in the playing position to begin playing.
You then need to know what notes each of the strings should be tuned to. Starting at the thickest string and ending at the thinnest the notes are E A D G B E. To remember this make up a sentence like Eddie Ate Dynamite Good Bye Eddie.
Another top tip is always tune up to the pitch you want to achieve, this will also help you stay in tune. Even if the note you are trying to achieve is sharp, slacked the string till it goes flat and then tune back up to pitch.
Do this for each string in turn. If your guitar is badly out of tune you may need to do this a couple of times untill each string stays in tune even when you have just finished tuning the other strings.