If you only have one size of pens then it makes total sense to use that. But if you have more (and Im guessing you have plenty) then consider what size to use. Of course it depends on the size of drawing you want to do. But a rule of thumb is that you either use the same size for outline and details, or use a thick for outline and thin for details.
How you create your lines makes a big difference. This is where you breath life into your drawing. If you create lines that are completely perfect you will get a super flat experience (in my oppinion) but if you get a little sketchy and breaks up the line a bit you will get a lot more life.
When you shade your drawing there is some great techniques to use. They are called hatching, cross hatching, stippling and just thickening the lines where you need them to be thicker (pretty simple).