-ar verbs use /a/ as their principal sound. For the closed historic group of -er and -ir verbs, the key sound is /e/. Everything else you do is identical: add n for they and you plural, s for informal you.
-ar: organizo, organiza, organizan, organizas
-er/-ir: vendo, vende, venden, vendes
Contamina · Contaminan · Contaminas · No venden · Vende algo · ¿Por qué no lo venden?
When building a sentence, take your time to understand who is doing what. Split it into two thoughts: first work out the verb, then work out the object.
I sell → vendo. I sell them → los vendo.
A sentence with an us, an it and a they in it could look hugely complicated if you took the whole package and tried to translate it. Think about the verb first, then the other pieces, and you get the sentence you want with confidence.
Where English says they sell it to me, Spanish identifies an it and a me and puts the personal one first. You are effectively saying they sell me it.
me / nos + lo / los / la / las + verb
Me lo venden · No me lo venden · ¿Por qué no me lo venden? · Nos lo venden · No nos lo venden · ¿Por qué no nos lo venden?
Algo is not one of these little words, so it comes afterwards instead: nos venden algo.
Words ending in a are feminine, with a few exceptions. Two more endings are reliably feminine, and words with them take la without exception.
-ity → -idad
Both la casa and the noun itself go plural together: las casas.
Vendo las casas · Las vendo
Once you have a word like electricidad, you can assume electric is probably similar in Spanish too. There is no rule for it, but you can assume it and try.
The skill is to get rid of the ending. The -idad is what makes it a noun, so la claridad gives you claro, not "clarido". Claro is used a lot conversationally, like of course, to show agreement or understanding.
Adjectives go feminine and plural too: generoso, generosa, generosos, generosas.
Worth repeating, because it explains persona. Persona is a feminine word, so a man saying it about himself still says persona. The word is feminine, not him.