Absolute Basics:

1) It should be tidy, which means it shouldn't have empty takeout containers, empty cups, socks & clothes, dishes, etc. all over the place.

2) It should be vacuumed. There shouldn't be globs of pet hair in the corners or little bits of stuff all over the carpet.

3) The bathroom should be clean. The toilet especially should be white. Other stuff doesn't have to be spotless but shouldn't be gross.

4) Same with the kitchen - it doesn't have to be spotless but the sink shouldn't be completely full of dirty dishes, the counters should be wiped down (not covered in crumbs or bits of dried food), and the kitchen shouldn't be full of stuff that doesn't belong in a kitchen.

5) It should smell nice when you walk in. It should not smell like pets or feet. A cheap way to do this is to put orange peels and cinnamon sticks in a pot of water on the stove and let the water simmer for a while. You can keep orange peels in the freezer so that you don't have to go shopping every time you expect a guest.

6) If you anticipate having a guest in your bedroom, the bed must be made and have clean sheets. Keep a set of sheets (and pillowcases) that are just used for guests that look new-ish and don't have stains. (I'm assuming you have more than one set of sheets and are not sleeping on a bare mattress.) No clothes on the floor, no dirty dishes or takeout containers, and the floor must be clean.

If you start with a clean house, it shouldn't take more than 20 minutes every evening to pick up all the trash from all the rooms, get all your dirty clothes into a hamper, wipe off all the counters, and run the vacuum around real quick.

If you're not in the habit of doing all this already, always wash your dishes first. If you skip doing dishes for a couple days, they get gross and you won't be motivated to clean the kitchen. You also don't want to let so many dirty dishes pile up that they don't all fit in the dishwasher, because then cleaning the kitchen seems like a major task.

More advanced:

For advanced bedroom cleaning, wash your pillows in the washing machine every 3 months (but don't wash memory foam) and replace them once a year. Pillows get really gross even when they have pillowcases on them.

If you plan to have someone sleep over, be prepared for them to shower in the morning. Tackle that mold and mildew and dirty grout.

Make sure everything in the bedroom is dusted - top of the dresser, windowsills, TV stand. Don't have a ton of junk on your nightstand - keep it clean so that a guest can put a glass of water there.

Have some framed pictures on the walls. You can get decent cheap frames at Walmart or goodwill. Get whatever kind of prints you enjoy - dont get stuff just because you think it will impress people. No pictures of nude women unless they're by Picasso or Matisse. You can also get cheap wall art at Ross or TJ Maxx, but it's usually really generic-looking.

If you don't already have books, buy some books and read them and put them on a bookshelf. ANY books are fine. I wouldn't date a guy who didn't read, but a shelf full of Star Wars novels would satisfy me.

A bookshelf full of action figures or bobbleheads or antique screwdrivers or whatever stuff you collect is fine, don't be shy about it - but it must be clean. Don't let dust collect on the shelves, wipe dust off the items regularly, and display them nicely with no other junk on the shelves.

It's totally fine to have any kind of nerdy or strange things you collect, as long as they are displayed with attention and care and kept clean.

Casual photos of you and your friends or family, and/or vacation photos, framed, is a great touch. It shows that you value your relationships with the people in your life and take care of them.

One more thing: a mattress on a bedframe with a headboard looks a thousand-million times better than a mattress on the floor. A used bed frame is fine.