Are you painting with oils? Acrylics? Watercolor? Each is a different technique for blending (watercolors blend with water, so do acrylics, but in very different proportions. Oils usually need to be thinned to blend right but I'm not sure how). Trial and error is your best teacher here. Your media type makes a different too. papers blend differently than canvas or linen or wood do.
Straight lines are easy, Use masking tape. You can do the same for curved lines too I suppose, tape off a larger area, draw your curves, and then use an exacto knife to remove the area you want to paint. Doing it with brushes, find yourself a nice soft angle brush or a line brush for those. Depends on your technique really and how much pressure you apply.
practice, practice, practice!