As you know, each blockchain starts with a genesis block and keeps building on top of it. Therefore, before running your private network, you first have to prepare your own genesis block by defining a genesis file. 

You can easily create new genesis files with a command-line tool called puppeth, which is a private blockchain manager that comes as a utility with Ggeth. Run puppeth and follow the configuration wizard. You'll need to answer a few questions as shown in the following screenshot:

When it asks you about accounts to pre-fund, give it the addresses that were created earlier. At the end of the process, save the genesis file as a JSON file, genesis.json: