First, you have three sentences here. So start by adding the proper punctuation. Learn what to look for when buying a mobile home. Most mobile homes come with land packages. Various necessities can be added to your loan.
Next, here is one way to change these sentences to passive voice, although I don’t think doing so makes them better: When a mobile home is bought by you, learn what to look for. Most mobile homes will come with land packages. Your loan will have various necessities added.
Passive voice gets technical quickly. But, basically, when the subject of the verb is doing the action, the voice is active. When the subject of the verb is being acted upon, the voice is passive.
Finally, if you work in Word, don’t take everything the grammar check says as Gospel. It isn’t.