Hola Esteban,
I don't understand your question very well. The preposition "a" can be used in many different instances, if you have a particular example or a specific question it would be of great help.
You are right in sayint that "a" is placed before nouns or pronouns, and it is also never placed before descriptive adjectives.
The preposition "a" collocates with several verbes, e.g. hablar a alguien (por teléfono), contagiar a alguien, conocer a alguien, ir a un lugar etc.
Exemples:
Juan le habla a su madre todos los días.
Mi hermano contagió a mi hija de gripe.
Conocí a Roberto en la fiesta de San Valentín.
Voy a la playa todos los veranos
But many other verbes don't require "a". You will have t get used to the verbes that collocates with "a".
Context is also very important with some verbes, e.g. "Conocer a alguien" (conozco a Alfredo) vs "conocer algo" (conozco el camino a mi casa).
I hope this helps.