New M4 Air Laptop... time to back up, Help a girl out please

I had the 2017 time machine my passport from the apple store and now I need something newer and bigger.

I'm reading comments from yall about reformatting HDDs and SSDs.

Help a girl out.

My external hard drive should last for several years, get plugged into my laptop about every 2-3 weeks and I don't want to have to do anything to it. Just plug and play.

My new mac is 500g. I have about 250 g in storage right now.

Could you just tell me what to buy? I have a best buy around the corner.

THANK YOU>