MOST forms, you add "s."
With words ending in s, z, ch, sh, and x, you add "es."
If the word ends in a "y," you drop the y and add ies. (family ---> families)
If the word ends in an "o," you sometimes add "es" and sometimes add "s." There's no rule whatsoever for this and they have to be memorized.
The question about irregulars (man-->men) is just that...they're irregular. English is made up of a combination of many different languages put together, so there's no real easy way to come up with rules.
I assume, from your screename, that you might be a person studying English as a foreign language. My apologies if I'm wrong. From many years experience teaching ESL, the worse thing you can do is try to remember all the rules. It won't help you at all unless your only goal is to pass a standardized test made for your specific area. (Taiwan has its own standardized test that is popular here that does not help people at all).
The real way to learn grammar is to become fluent to a level of English that you have already seen these grammar points several times and are familiar with them. With that, you simply need someone to point them out to you. Trying to learn it and apply it simply won't work and it is backwards from how we learn grammar. Keep reading, writing, and listening to English and let the grammar come more naturally.
Matt is right. Just dive in and don't worry too much about rules, especially when you are first learning.
In addition, here are some websites that may be of help:
Click on STUFF FOR STUDENTS for a variety of help topics
You can read past blot posts or post a question for yourself.
There are many topics here. You can also test yourself (and learn more) by going through all the Grammar Blast quizzes, no matter what the grade level is.
Both of these have really useful sections.
Sorry -- that last link should be this: