There doesn't seem to be any hostile mobs spawning in Minecraft: Pocket Edition

I"m playing survival mode, but there seems to be no hostile mobs, such as creepers (which is kind of boring).

How can I make them appear?

Make sure you're not on peaceful mode - this can be selected in the options. Hostile mobs will not spawn in peaceful mode.

Mobs also won't spawn on half blocks/slabs or transparent blocks, so if you're surrounded by a huge area of glass there would be no valid spawn location.

If there is a large, dark area nearby, hostile mob spawning might all be concentrated in that area - search around for caves or overhangs and light them up to force spawns to happen away from the location. (This is often stated as the cause of not being able to find hostile mobs, e.g. on this Minecraft Forum post)

Conversely, if you've lit up the whole surrounding area to above light level 7, you might be stopping spawns - hostile mobs can only spawn in areas of light level 7 or less.

You can reset the spawns in your world by switching to peaceful mode to remove all mobs, and then switching back up to normal/hard to make them spawn again.

As for attracting a creeper once you've found one:

The Minecraft Pocket Edition Wiki page on Creepers states that Creepers will follow you from a range of sixteen blocks away.

You have to make sure It is not in peaceful mode or the mobs will not spawn! And you have to make sure that the torches amount is not above 8 even though you are in Normal/Hard mode, they will not spawn in that Area.