With your 'tightish' cardigan you could either put a pencil skirt, on the knee or below for the 'sexy secretary'/ Joan in 'Mad Men' early series. Red lipstick, black mascara and eyeliner, heels, nude tights, pearls maybe. Or, cuter, think Sandy at the beginning of 'Grease' . Your cardigan is over your shoulders. Little white cotton short sleeved blouse, flared skirt, wide belt. Plimsolls (Green Flash!) ankle socks or ballet pumps. Pearls again. Pony tail.
Another 'look' could be inspired by Sandy at the end of 'Grease' ( 'You're the One that I Want')
Black leggings, black 'leather' jacket, black T shirt, coloured stilettos.
I think your look will be dictated by what you can find in your charity shop trawl. Good luck!