How to Get a Nice, Round Home-made Corn Tortilla
I roll them into a ball and use a cast iron press, similar to this one:



I think the official name is a "tortilladora". You can find them online or at restaurant/cooking stores for $10 to $20. 

I wouldn't worry too much about getting them perfectly round. Store-bought tortillas are perfectly round because they're made by a big machine. 

If you're still having problems with a press, I would look into a different recipe or maybe use more shortening/lard.