I agree about focusing on the lines for now … but don’t necessarily only use ballpoint or other pens.Some people do great with ballpoints but I have trouble because you have to go over the lines a lot before you can get any real lineweights. It’s also very hard to make curves / ellipses bolder without getting super hairy. A lot of people recommend doing linework exercises with standard Sharpie – it makes you commit to each line (and can be very fast). Markers can be great for linework alone.
If you see Jeff Smith’s (Concept Sketches - Linework - Single concept per page by Jeff Smith at Coroflot.com) stuff like the image below, you can imagine how quick it can be … IMO quicker but cleaner than you could do with a ballpoint in the same amount of time. You can also get the same effect with felt-tips pens or even cheap-o Crayola Supertips (though they might bleed if you shade later).
