That is the standard advice for newbie writers. Some may read that sentence and draw a blank; they might think they know nothing. Not true!
Start with paper and pen; write down any and everything you have seen, heard, experienced, read about, took part in, discovered, played, applied for, cooked, baked, made; the list is endless.
Take your list, pick one subject and flesh it out. Describe what you see, smell, hear and taste and touch. That’s what you want your reader to be able to do.
You can use old photographs of past vacations or places you plan to visit; describe your perfect time there.
When I am stuck for ideas, I start to write what I know. When I pull images from my past, I tweak them and create fictionalized places for my characters to live in.
On one of my travels around the city, I noticed an abandoned shack that used to be a stripper club, sitting off the road in a dense patch of weeds. That is what I know.
My mind went crazy. What can my characters get up to in there; what brings them there in the first place? Are they looking to buy the land as an investment or, are they running from someone or something and end up there in their frantic effort to find a place to hide? Do they struggle to open the rusted, metal door only to discover the place dusty, dank and dark. Do they wander around and suddenly taste bitter bile rise in their throat when they see their haven is already inhabited by human-sized vermin with fangs and bloody eyes?
Let your imagination loose but start the ball rolling by writing what you know.
All my best,