In most cases, you should be able to replace corn syrup with sugar. The usual rule is as follows:
1 cup corn syrup can often be replaced by 1 1/4 granulated sugar (or light brown sugar) plus 1/4 of liquid (use water or whatever liquid is specified in the recipe you’re using). (there are homemade recipes out there that include cream of tartar and boiling to soft ball stage...but that breaks down the sugars and ends up not being a healthier alternative to store bought corn syrup.)
Lyle’s Golden Syrup (use in same quantity as corn syrup) -- Lyle’s Golden Syrup is widely available in the UK, but possibly harder to find in the United States. Decent sized supermarkets may stock it in the British section of the International aisles, I suspect.
Honey (use in same quantity as corn syrup) -- honey is quite a bit sweeter than corn syrup, but if the recipe doesn’t call for a lot of it, this may be an effective substitute.
Molasses (use in same quantity as corn syrup) -- molasses is darker, obviously, and also stronger in flavor. It’s also less sweet than corn syrup, so is probably your last resort.
If you’re baking something like cookies or bread, I’d suggest you try the honey substitute. Honey, like corn syrup, is hygroscopic. Hygroscopic substances attract water molecules from the air. What this means for baking is that cookies and bread baked with honey rather than sugar will tend to stay softer longer.