I have a child 8 years old, born in Colombia.
If I get the Green Card, can I petition him? I don´t want to apply for him before I get a Green Card.
Yes, you can apply for a green card for your child. If your child is in the US on a valid NIV status you can file an I-485 for him. If he is outside the US, you will file an I-824 to start consular processing for him at a US consulate abroad.
For your application, if you will be in the US on a valid NIV status and filing an I-485 you must be able to show that you will not become a public charge to the US. The best evidence is an employment letter and recent pay stubs. If you are not employed you can provide evidence of your assets in the US (e.g., recent bank statement, deed to property in the US, etc.). If you do not have any assets in the US you will have to provide an Affidavit of Support.
If you will be applying for your green card from outside the US through consular processing you do not need an Affidavit of Support since your green card is through employment-based immigration.