I am a Colombian professional, married to a professional.
We have two children.
We want to know how difficult it is to get an American visa to work in your country and if it is necessary to have an employer before applying for the visa. In that case what is the most effective way to look for that job?

We have received many questions similar to this one.
The simple answer is that as a general rule, you must have employment arranged first and then you apply for a visa with the employer listed as your sponsor.
You cannot get a work visa before you have a job.
There are some narrow exceptions for highly skilled persons of "extraordinary ability" and exceptionally qualified persons who are doing work in the national interest of the United States.
Persons in these categories (such as, for example, top athletes or artists or scientists doing highly regarded medical research) are the only ones who can file their own petitions.
You are on the right track toward finding a job if you are reading.
You should post your resume and read the articles regularly to find tips on locating a suitable position for your qualifications.
Once you are offered a job interview, you will be able to come to the U.S. as a business visitor for the interview and then return to your country until your employment (H-1B) visa has been approved (typically 4 to 6 weeks).