I advise you to start drawing on blank sheets of paper. I understand the convenience of lined paper, but it's for presentation reasons. It's slightly distracting for the drawing to have lines going across the drawing.
I suggest you use imgur to post your images, since it will provide a high-res and low-res version upon uploading, the low-res version can be used for forums such as mangaraiders. Make sure you always flip your drawings so that it's facing the right way up, once again for presentation reasons.
Now onto the drawing. The more copies you do of a certain artist, the better you're going to get at copying that artist. You might sometimes feel that when you start copying someone with a relatively different style, you're going to have a harder time. But I advise you to switch sometimes, as it improves your power of observation and ability to recognize line relationships in drawings. Some additional things to think about is what kind of line quality the artist is using in their drawings, and why are they doing it? What I mean by line quality is where is the line thick, where is it thin, where does it break altogether and re-appear. (Also, when drawing with pencil, line quality is also how light or dark the line is). And try to replicate this line quality. This might not be immediately helpful, but it will help you gain intuition on how to use line, which will help you in the future.
Also, think about posting more drawings from imagination. That is eventually what you're going to be doing the majority of the time as a manga artist. So it's important for us to know how good you're at it and what you need help with so we can help you improve it.