Restrict espers by character.

1) What does an org instruction? Setting the offset where to write?

Exactly. It's the address I wish to start writing or altering code.

2) What does the DB and DW instruction means? I noticed they are used in tables or strings...

They're used to simply insert raw hex. DB inserts a one-byte value (since each letter in a string is one byte in length), DW for two-byte values (for the two-byte addresses in pointer tables), and DL for three-byte values.

3) You mention the word editor... Is there a good editor to write ASM files or you simply use something like notepad ++? A program with a compiler that would find syntax or code mistakes for the 65816 would be really good. Is there any?

I used Visual Studio, then switched to Notepad++ when I realized it highlighted better than VS. I used xkas to compile, but if you prefer GUIs to command lines SNEStuff will also compile with xkas. Both have pretty thorough error-checking.