I start to create my own cryptocurrency and start forging what's next ?

Remember this is only for testing, so when you start publishing your copy you will need to comply with the JPL license.

To run a test do the following:
In:
nxt-clone \ conf \ data \ genesisAccounts.json
Enter this:

{
     "balances": {
          "NXT-NZKH-MZRE-2CTT-98NPZ": 30000000000000000,
          "NXT-X5JH-TJKJ-DVGC-5T2V8": 30000000000000000,
          "NXT-LTR8-GMHB-YG56-4NWSE": 30000000000000000
      },
      "publicKeys": [
          "bf0ced0472d8ba3df9e21808e98e61b34404aad737e2bae1778cebc698b40f37",
          "39dc2e813bb45ff063a376e316b10cd0addd7306555ca0dd2890194d37960152",
          "011889a0988ccbed7f488878c62c020587de23ebbbae9ba56dd67fd9f432f808"
      ]
  }

In:
nxt-clone \ conf \ data \ genesisParameters.json
Enter this:

{
"genesisPublicKey": "1259ec21d31a30898d7cd1609f80d9668b4778e3d97e941044b39f0c44d2e51b",
   "epochBeginning": "2021-11-24 12:00:00 +0000"
}

To boot.
Alternatively you can enter your own addresses with public keys

The passphrases to enter the wallets are:
0, 1, 2

I have already passed this stage, I saw that for Monetary system it was normal to have nothing, but what interests me now is to find tutorials to submit my crypto to the market.

Forgive me, could you be more specific?

Of course, several unknowns come to me:

1 / The height of the blockchain is 38,000
How long should I keep forging?

2 / How to make my crypto accessible to the outside world

Or it's easier than I think, and make a simple transaction to an external wallet.

You should forge forever, you also need a team that keeps your project healthy in appearance, software, marketing, social, legal etc.
Unfortunately for your project to be successful nowadays, you should have a number of things.
If your project is interesting you could consult with Jelurida, she has already collaborated with various projects started.
If you wish, there are many tutorials on the internet.

What do you mean by external wallet?