Related to this question, how can I set up or select no calligraphic brush? Thanks all
Your question is rather short on detail, so this is a guess. If this isn't what you want to know, then edit your question to add more details.
If you want to draw basic paths, without a brush setting, then don't use the Brush tool.
In the Appearance Panel make sure the option "New Art has basic appearance" is deselected.
Choose the Pencil tool (not the Brush tool).
Set the stroke size, select "Basic" from the dropdown.
Begin to draw - ignore the toolbar along the top, it will change back to display a 5pt round brush. Just keep drawing - all your paths will be just basic strokes.
Special thanks of course must go to Adobe for making this spectacularly unintuitive, and just plain silly. The behaviour showing the switch from basic to 5pt round drives me crazy too. Just ignore it.