Standard tuning gives you a balance between playability and musical range, which makes chords, scales and melodies easier to play across the fretboard. It's the tuning used in the majority of guitar lessons and instructional materials, so it's the right place for beginners to start.
For standard tuning, many electric players use .009–.042 or .010–.046 sets. Acoustic players commonly use .010–.048, .011–.050 or .012–.054.
If you plan to tune lower than standard, you'll usually want a heavier gauge so the strings don't feel too loose. See our articles on Drop D, D Standard, Drop C, C Standard, Drop B and Drop A for gauge suggestions for each tuning.