Commas are quite handy little punctuation marks though people often have trouble knowing how to use them correctly. There are two ways in which a comma should be used.
1. To separate items in a list. When you are listing objects in a piece of writing, each object or item should be separated by a comma so that the reader is able to identify the fact that they are separate, individual items. Example: You must buy bread, milk, sausages, tuna and bananas today.
2. A comma should be use to break up a longer sentence into smaller chunks. It indicates a pause in the sentance which is shorter than a full break such as a fullstop. Each segment of the sentence should be linked to the other segments and relate to the same idea.
When deciding where to put your comma, it is helpful to read the sentence, aloud or in your head. You will notice the natural "pause points". If you take a breath in the middle of the sentence, put a comma at that spot. Example: I went to the store to buy bread, however, they were out of stock.