It appears that you are a casual illustrator/beginner. I would recommend that you start with focusing on your understanding of anatomy. Your basic shapes are in the general proportions of a muscular system, though your specific understanding of musculature is lacking. You should study the art of Dale Keown. He's got all the musculature, shading and proportions down to a masterful science.
Your line-work is confusing. Once you understand anatomy better, you'll understand a bit better how to emphasize where to place the proper shadows and detail. Your variation in line thickness is lacking as well. The logic behind placement of thick vs. thin is simply that you A.) Generally, place thicker lines as the objects get closer to camera, and thinner as you travel backward in distance. B.) Specifically, you'll better understand where the details of the thick vs. thin lines will go, once you understand the actual structure and shape of the objects you are illustrating.
There are many many factors that you must be aware of when illustrating a bold and appealing image. Before you gain awareness, you must learn the fundamentals.
Everything has an underlying structure. In order to properly represent anything in an illustration, you must have a thorough and clear understanding of it's structure. Organic objects like humans, animals, plants and such, are far more complex to understand than buildings and furniture, because organics have countless underlying shapes that compose the whole object -- and they are constantly shifting in shape depending on the pose, angle of camera, action, etc... whereas inorganics like buildings and furniture and cars (though each of these can also have more organic design, and so be more complex in structure) have definitive shapes, and therefore easier to comprehend.
There are many books and online resources to gradually learn such things. However, I recommend that you learn from real life and realistic art, instead of specifically the art of comic illustrators or cartoonists -- simply because every illustrator has his own style, and limit to his understanding of structure and the way he/she represents this understanding in the art... whereas life-drawings (though boring), will teach you the structure you need to understand, and as you progress, you will naturally learn how to apply style and create attractive art.
This is my general review of all your images below. It shows that you have the interest, and underlying talent, but you need to focus and educate your eyes and mind to understand the structure of life, before you can effectively communicate your imagination through visual art.
Work hard. Don't loose focus. Be obsessive. Don't give up. Stay the course.