Every buyer will pay a home inspector to go through your house with a fine toothcomb. They will check the roof, the plumbing, the electric system, the windows and doors, the heat and air conditioning. To make a good impression on a buyer and to save trouble later, you should go through these things yourself or pay an inspector to do it. You don't want a sale to fall apart because the inspector found something of which you were unaware.
You should fix things within reason. If a sink needs a 50 cent washer, or shower tiles are missing grout, get them fixed. Have the A/C serviced. It will cost little but you will be able to show the buyer the receipt to assure them that you have maintained the property. If there is a septic tank, have it inspected and pump it if necessary. Have a licensed contractor inspect your roof. It may look good from the front lawn but you don't want the inspector to find a broken shingle in a spot that was hard to see. Make sure the doors and windows operate easily and do not squeak. A drop of oil on a hinge will make a big difference.
If your preliminary inspections expose a problem that is too expensive to repair, by state law you must disclose it to the buyer. They'll find it anyway, so why not show that you are honest and tell them above board? For instance, if you find that the garage door opener is not working properly, let the buyer know. You can then say, "I'll give you a $250 credit at closing and you fix it." That gives the buyer confidence that you are not trying to hide anything.
One of the best and cheapest ways to make a house look great is fresh paint. Take a look around and be honest. Do the rooms of your house need painting? If so, you can do it yourself and it won't cost a lot. Do not pick colors that are unusual. The best choice is off white. It makes a room look bigger and a new owner can easily paint over it.
Finally, clean the house or hire a cleaning crew to do a one-time whole house cleaning. Include a carpet cleaning because that will help remove old odors and freshen up the home. If the carpet is very old or outdated, give the buyer a carpet allowance. Take a look at the exterior. You should clean up the yard and toss any junk or old lawn furniture. Make it look neat and clear of debris. Mow the lawn and trim the bushes. A lawn service might be a wise choice while your home is listed for sale.