There is no graphics tutorial. Graphic quality is very subjective and depends on many different things. There are technical things that influence the quality of graphics, like the texture resolution, amount of polygons etc. but there are also a lot of other factors, like the artwork and the concept of a game. So your question cannot be answered by a tutorial or a forum post. You should learn the basics. Once you got to know the basics you will be able to answer the question you asked for yourself. That's why I recommended the tutorial.