You have to do what everyone else has done. DISCOVER a seed that generates a world that has features that appeal to you. Whether that is unusual terrain, certain biomes, number or size or closeness of villages, treasure to loot, caves, ravines, or anything else that tickles your fancy.
Any text you type into the seed entry box will produce a unique world unless you type a "0" (zero). That and not entering anything will cause the MineCraft world generator to make a, for all practical purposes, random seed.
All seeds, as used by the world generator, are numbers, not text.
You can find out what number was used to generate the world you are currently in by typing the command "/seed" and MC will respond with the NUMERIC seed that was used.
Numeric seeds can be any integer from -9223372036854775808 to 9223372036854775807 which is a total of 18446744073709551616 different worlds. Multiply that by 3 to allow for default, large biome, and super flat worlds. Note: those numbers are 19 digits long, not counting the beginning plus or minus signs. (20 for the total, for you nit pickers)
If you type ANYTHING into the seed box other than +,-,0,1,2,3,4,5,6,7,8, or 9 (that means no unwanted/unseen spaces also) MineCraft will use the Java hashcode() routine to turn your text into a integer between -2,147,483,648 and 2,147,483,647 which the world generator will then use.
So, your choices are:
1. Type in nothing (or 0) and get a random world.
2. Type in up to a 19 digit number with or without a leading plus or minus sign.
3. Type in any text (up to 32 characters) that is easy to remember, or not, your choice.
In all cases, you can see the numeric seed that was used by using the /seed command. Entering that revealed number at a later date to generate a new world should give you an identical world to the current one.
Please note that the /seed command is disabled for users of multi-player server worlds to prevent users from cheating.
If you are wondering if you can replicate all man-made structures added in other people's worlds, the answer is no.
Seeds affect natural terrain generation, in that the terrain generation uses random-ness to pick how it looks. The seed manipulates the chosen numbers and their order, which gives consistent and shareable "random" terrain. But nothing more.