Can you describe like I'm 5 how can I get the first address of the first account with this tool according to BIP-0044 given I know the xpub?
It should be m/44'/0'/0'/0/0
If you have the extended public key corresponding to m/44h/0h/0h/0 (note the syntax in BIP32 has changed) in a file, say test.xpub, you can simply:
$ bip32gen -i xpub -f test.xpub -o addr 0
1NaKvDgeVnXqvTpFzzngmH1RG9cN3BkVfh
This translates as "starting with an extended public key stored in file 'test.xpub', generate the address associated with child number 0, and write it to stdout". (Address shown is from an xpub from an arbitrary wallet seed).