Well, AS3 has some pretty function that manage to do this, but sometimes, the documentation gets you wonder how practical it is.
So here are some examples for common operations with Strings:
var myString:String="Always speak the truth, think before you speak, and write it down afterwards.";
1. Show the first character
2. Output the first 6 characters
3. Output from the sixth character
4. Slice the string from the 8th to the 12th character (basically will to the same has the substring)
5. Search for a word and output it's position
6. Convert all letters to lower case
7. Convert all letters to upper case
8. Length of a string
9. Concatenation... or adding something to the string (String are not like numbers, you can't add an "a" to a "b", so the operation "a"+"b" will result in "ab")
10. Replacing text with other text (this might get trickier)
(this will replace the word "Always", wich is between character 0 and 6, and replace it with the word "sometimes")
Hope it helps someone.