Paper and something to draw with - like a pencil, pen, or marker - is the simplest thing that might work. Maybe splurge on a scale if you need accurate plans and a compass if the wheels you draw need to be round.

[Caveat: I'm assuming you actually want custom bikes and not custom designs (assuming that the goal is to make bikes and not plans or models) and I believe that the best way to start building bikes is building bikes not learning to use software. YMMV. Good luck.]