What is it called when you ...?

Question

i just had it in my head and forgot!

when you take a word and write something for each letter like... sam

S- Someone who always helps me

A- Always there

M- My Best friend

Answer 1

You are either thinking of "acronym" or "mnemonic" but I think in your example it would be mnemonic, because acronyms usually (maybe always) have one letter per word, compared to mnemonics which are any convenient memory way to remember words or phrases.  See the two definitions and decide for yourself:

http://en.wikipedia.org/wiki/Acronym

http://en.wikipedia.org/wiki/Mnemonic

Answer 2

Acrostic?

Yes, acrostic. http://en.wikipedia.org/wiki/Acrostic

See the definition and decide for yourself.

It's also called a name poem sometimes, yes