Fakes are less common with common coins, less expensive and lower grade coins. But there are still fakes out there for those. Don't worry about it too much, because there are far more real coins out there than fakes, for most coin types.
For example, go to
http://www.vcoins.com/ and then search for Constantine I. At the top should be a sort by lowest prices. You can look at the coins, most of the time they get better as the price gets higher. Here is one that is decent for the price. Since you don't want to spend a lot right now I found a cheap one with decent detail for you to look at.
http://www.vcoins.com/en/stores/imp...Default.aspxOnce you manage to purchase a bunch of coins, you'll know most of the time what is real and what is not. If you have doubts, don't purchase the coin. You can ask here as well.