Im not sure how bad graphis your talking about so posting a picture will help, but making graphics look as good as the onse you saw require good lighting, shaders, High ressalution textures and good models.
lighting is fairly simple. With the right placement of lights, right color, and right brightness this can make your game so much better in terms of graphics and imersion.
Shaders is more complex and i don't even fully have a grasp so i will skip this and let someone else tell you about that.
For high resolution textures you can download, and/or make your own high resolution texture and apply it to the model.
With models the easiest way is to but a model pack. or make your own models. for model packs i recomand
http://www.thegamecreators.com/?m=view_product&id=2038
http://www.thegamecreators.com/?m=view_product&id=2037http://www.thegamecreators.com/?m=view_product&id=2287
I got them all together with a bundle that to my knowlage is no longer avaliable and i dont regreat it, but of course it also depends on the typs of games you are making.
here is a list of model packs
http://www.thegamecreators.com/m=view_product&id=2001&page=ModelPacks
Hope this helps.
