How do you spell weiner

In many cases, you can't nail down the spelling of a word without knowing
what it means.

You didn't tell us what your word means, so there are different possibilities. 
Here are a few:

-- In old German, a wagon driver was a wagner (VOG-nair) or weiner (VEIN-air).
As the Yiddish language (spoken among German Jews) developed from old high
German, some of them used the same word 'weiner' to mean 'one who makes or
sells wine'.  The word came to the New World as a family name, spelled "Viner",
(as in my first high school crush).

-- The ancient city of Vienna, now the capital of modern Austria, is called "Wien"
(VEEN) in the languages around there.  A person who was born or raised there
is called a Wiener (VEEN-air).  Also, a small sausage that became popular there
was also called a Wiener. That's where we got the slang term 'weener' for a hot
dog or anything that resembles one.

-- A little kid who whimpers and whines all the time is called a whiner.

So the spelling really often depends on what your word means.  That's one
reason why, in a spelling bee, they always give you a sentence along with
the word.