How can I cover up an unsmooth surface so I can paint it?

Michelle, this depends on what it is. Can you sand it? If it's raw wood, then use a stain that soaks in. Is it drywall? Then you can run a skim coat of drywall compound over it. Concrete or masonry? Use masonry stain.

Can you post a picture?

Can you provide some pictures?

You don’t say what kind of surface it is wood,tile,vinyl,Sheetrock,

Sand if at all possible. If that isn't possible then you can use some sheetrock mud and a trowel to apply and smooth over the surface. When it is dry, then use a damp sponge to smooth. You might have to re apply a couple of times, I use the rule of 3.

Michelle, As you have seen by the answers, we need more information. Then you will get the correct answers. 🌹💖😘