My truck wont start it says the battery is low

The battery needs to be checked, may be low on charge, may be old & bad. Also the charging system needs to be checked to see if it charges the battery. Make sure the battery cables are in good shape, clean and tight.

its a new battery

You need a alternator

Then have the charging system checked like I mentioned, make sure the battery cables are in good shape. Unless the battery is draining down while it sits. Then it would have to be checked for a parasitic draw on the battery.