Grammar and punctuation can be pretty hard to do on your own. Sadly, in the US, children aren't always taught the proper use of grammar and punctuation in primary and secondary education. Or, perhaps, you're not a native English speaker, or have yet to finish primary or secondary school.
Good news, fair folk. I have compiled a number of my notes from my Writing 121 class on grammar and punctuation. This class was taught by a long time PhD professor and professional editor. While this may not be a complete guide, it will help you cut down your mistakes by at least 25% (not guaranteed, numbers may vary).