Just started and interested in ancient coins...how do I know if the coin is legit?

If you don't know, make sure you buy from someone who does. VCoins not eBay, for example. eBay is for when you know what you are doing. There are plenty of good dealers but where you live would determine the best to go to.

To know yourself, spend some time looking through auction results of the coin type you are interested in until you can appreciate the style and what does and does not look right. Soon, you will start spotting fakes on eBay, although even the best can't spot them all. Style, weight, edge seams, soapy devices and little perfectly-round holes from casting, very flat fields and a lack of flow lines (radiating from the legends) from pressing, die matches to other fakes - all help spot a fake. But it's just a matter of percentages - you want to make the chance it is fake as low as possible.

You could go over to the dark side and buy an NGC slabbed coin. They don't guarantee it is genuine, it will cost a lot more than it should (unless it is a very expensive coin) and you can never touch your piece of history, but someone knowledgable looked at it. However, you can get the same but with a guarantee if you buy from CNG or many European auction houses, and without the slab.