I have little fighting skill. How can I get rid of a Wither?

As of Minecraft 1.11, you can kill a wither boss with a relatively simple command: /kill @e[type=wither] (or /kill @e[type=WitherBoss] between 1.8 and 1.10). This will literally do what it says on the tin, and kill all wither bosses in the world (hopefully just one).

If you're not in a version after 1.8, you can also set the difficulty to peaceful with /difficulty 0. If they had any pet creepers or anything like that, you can spawn them back in and give them name tags again using spawn eggs, since they will also go away upon setting the difficulty to peaceful.

Unionhawk's answer is the easiest. But if you don't want to eliminate all mobs there's an alternative. Get into "godmode":

/effect Player_name 1 50000 5
/effect Player_name 5 50000 255
/effect Player_name 8 50000 15
/effect Player_name 10 50000 255
/effect Player_name 21 50000 50
/effect Player_name 23 50000 255

Now you can kill the wither with a single hit. Your jump and speed are also enhanced in case you want to do this in survival.

Once done, this will bring you back to your normal state:

/effect clear Player_name

Commands explained

/effect Player_name 1 50000 5 #Move faster
/effect Player_name 5 50000 255 #kill with a single blow
/effect Player_name 8 50000 15 #jump higher
/effect Player_name 10 50000 255 #Fast life regeneration
/effect Player_name 21 50000 50 #Extra hearts in health bar
/effect Player_name 23 50000 255 #Never get hungry

In the jump command, the "15" may be changed depending on your desired jumping height. If you are in creative, you might not need the jump command at all and you can just fly towards the wither instead.