Cardschat is a good place to start but there are others as well. If I were you I would ask myself do I want content specific to a type of game or just all content? Meaning am I going to play mostly tournaments, cash, or both. From there do a little research online and you will find multiple training sites. I have listed two below that I recommend looking into. In the end it is your money so you will have to decide which site you think is the best since there are many out there.
Cardschat is a great place to start, there is so much material here it will take you a while to work your way through it. Also post your hands and questions you have about specific spots that you have questions or difficulties with.