Yo and lo are very different. Lo would really replace the English "it"

For the example above: Lo voy a buscar really is, (Yo lo voy a buscar).

However, we do not really have a word for it in Spanish, so don't take it literally. It just replaces the function.

For example: If I said, "Yo voy a buscar" then the person would ask..."buscar qué?, but if you say "Yo lo voy a buscar" you have already been talking about "it" and it is understood what "it" is. Because of  conjugation, "yo" is not neccessary.

you can use "lo" when you are referring to a question about you, example: lo voy a buscar? 
Translated : I'm going to look for?, however if you use "I" in the same sentence sounds more like a verb , like an action

example : I will look

so the difference is that " what " is to ask about something that you do , in the future tense , and " I" is to indicate something you did or is doing these past tense , present tense.