There are two types of datablocks. an instance DB assigned to a particular FB and a global DB. A global DB you make by yourself and you can use the values anywhere you want.
To make an instance DB, first you make the FB. Add the FB to the Blocks folder. Open the FB. Assign any variables in the declaration portion at the top of the block. These will become the "pins" on the FB when you call it from another block. Close the FB.
Open OB1 or whatever block you want to call the FB from. If you're using ladder logic, insert a network, open the FBs folder in the left window, and drag it onto the network. it will show up with three red ??? on the top. Make you assignments to the input/output pins. Type a new DB number in place of the three red ???. When you hit return it will ask if you wish to create the instance datablock. Tell it yes and the datablock will be created.