What are the most commonly use features in solidworks to model a Handicap tricycle frame and What are the best starting point?
Start with the wheels, make an assemble and put the wheels in the required location. than use these wheels as the base of the model and build the frame "around it using the centers of the wheels as the base.
Design the frame IN the assembly (right click -edit part) so you can see what you do in relation the the existing part in the assembly.
Start 3D sketch. Start from the arrow. Draw line red xy-directions. Press tab once to draw line to z-direction at second arrow. Add dimensions. Fillet.
Make same for green and yellow lines.
Add work plane normal to line end (Project geometry.plane.click line,click line end). Draw circle on work plane (pipe diameter) to origin. Sweep.
Mirrow the pipe frame.
Voil'a, it's a start. Have fun.