A note about gauges...
If you're new to the guitar scene you may be confused by people saying things like 'I always use nines'. Guitar string gauges are usually described by their thickness in thou' (thousands of an inch). When guitarists talk about 'nines' or 'eights' etc they are referring to a standard set of guitar strings, with the first string being nine thou' or eight thou' thick.

The best all-round beginner gauges are either 10s (e.g. Ernie Ball Regular Slinky or Rotosound Yellows), or 10s (Super Slinky or Roto Pinks).

It's not that simple, however, as there are many variables in guitar strings not just the thickness of the first string. Most major string brands have a range that allows you to match the top and bottom strings to suit your style... so if you like to easily bend your high strings when playing lead but pound the bottom strings when playing rhythm then you can chose a suitable combination.

As a rule, heavier strings give a fuller guitar sound, but are harder on the fingers for beginners and are more difficult to bend when playing lead. Lighter guitar strings are easier to fret, better for expressive bends, slides and vibrato effects.. but give a slightly thinner sound and break more easily! In my experience, beginners who use 'eights' often break their E string!

It's generally assumed that the first and second string will be 'plain' (just a straight bit of wire), whereas the third to sixth strings will be 'wound' (a thin bit of wire in the middle tightly wound with a wrapping of even thinner wire to make up the total thickness). Where this generality is broken, it is indicated by p for plain or w for wound, added to the gauge. Again, you will hear guitarists say things like 'I prefer a plain G' - an example of this is Ernie Ball Beefy Slinkys, where the 3rd string is '22p'.

Here's all the Ernie Ball gauges available...
2215 - Skinny Top Heavy Bottom 10, 13, 17, 30, 42, 52
2220 - Power Slinky 11, 14, 18p, 28, 38, 48
2221 - Regular Slinky 10, 13, 17, 26, 36, 46
2222 - Hybrid Slinky 09, 11, 16, 26, 36, 46
2223 - Super Slinky 09, 11, 16, 24w, 32, 42
2225 - Extra Slinky 08, 11, 14, 22w, 30, 38
2626 - Not Even Slinky 12, 16, 24p, 32, 44, 56
2627 - Beefy Slinky 11, 15, 22p, 30, 42, 54