Wipe everything down with wax/grease remover or hot soapy water. usually if your frame is nasty i will opt for the water with a pressure washer. Blow all the nooks and crannys out with an airhose to make sure its dry. Then use red scotchbrite to scuff everything down. This lets the paint have something to stick to. Then wipe it down again with wax/grease remover, aka prep solvent. Blow it off again. Then paint.
Dont forget to cut the unwanted tabs off the frame to make it look a little better and ise a little primer on the bare metal before the scuff stage