This is because depending on the type of action and verb, a different particle must be used.
Let's remember that the particle Wo always takes a subject, and has a common structure of:
Subject は / が Object を Verb
NOTE: Let's remember that the subject can be omitted to avoid redundancy, so we might end up seeing just a sentence with "Object を Verb"
The type of verbs that carry the particle を are called "TRANSITIVE" verbs.
That is because: there is a transfer of an action from the subject to the object.
Or how we would know it in Japanese 他動詞「たどうし」A verb done to other things
Good examples that use the particle Wo are the verbs "To Eat" and "To drink"
(わたし は ）すし を たべる
Watashi wa sushi o taberu
(I) eat sushi
(わたし は) みず を のむ
(watashi wa) mizu wo nomu
I drink water
Now, let's talk about the particle "Ga"! Well... Easy, there's NO OBJECT! And this happens in pretty much two case scenarios:
1) The object becomes the subject, who then performs the action by itself (The light turned itself off -- maybe because the bulb burn out?)
2) The action was previously performed (The light is off... maybe somebody turned it off a few minutes ago, yet that doesn't really matter)
Now, as opposite from being transitive is "INTRANSITIVE" verbs! and these can be taken by particles such as the "Ga" particle.
Or how we would know it in Japanese 自動詞「じどうし」A verb done by itself
SOOOO, what is the biggest problem with these two particles? The main issue is that the verb changes (most of the times) depending if your sentences is transitive (wo particle) or intransitive (ga particle). For example
(私 は）まど を あけます = I open the window
まど が あきます = The window is open
In this example, the verb "To Open" is
あけます (formal) or あける (informal) if the sentence is transitive = open something
あきます (formal) or あく (informal) if the sentence is intransitive = something opens
Now let's take a look at a couple more examples,
(私は) まど を しめす ＝ I close the window
まど が しまります ＝ The window is closed
(私は) でんき を けします = I turn off the power
でんき が きえます = The power is off
(私は) でんき を つけます = I turn on the power
でんき が つきます = The power is on
(私は) でんわ を かけます ＝ I make a phone call
でんわ が かかります ＝ The phone rings
(私は) ほん を ならべます ＝ I line up the book
ほん が ならびます = The book is linking up
(私は) くるま を とめます = I park the car
くるま が とまります = The car is parked
I hope these examples gave you a better idea of the difference between Wo and Ga.
This "Transitive" and "intransitive" verbs topic can get very complex, but this post is just meant to give you a brief idea of the difference. This is a topic that is mentioned to beginners of the languages, so a very important one to start practicing.
Now the question is, Is there an easy way to remember the difference between Transitive verbs and intransitive verbs? Unfortunately there's no easy "hack". The best way to learn them is by pairs and in context (that is in actual sentences).
