I want to add random string as token for form submission which is generated unique forever. I have spent to much time with Google but I am confused which combination to use?

I found so many ways to do this when I googled:

```
1) Combination of character and number.

2) Combination of character, number and special character.

3) Combination of character, number, special character and date time.

```

Which combination may i use?

How many character of random string may I generate.?

Any other method which is secure then please let me know.?