Sisä means inside. Menen talon sisälle = i'm going inside of the building. Sisältä is used when you wanna say: you came outside from inside. But acctually you cannot translate finnish into english because finnish language has another system. Just accept it and remember it :))))) talon sisällä/sisälle/sisältä is used just for emphasizing the place.

The word "sisälle" really has the same meaning as "into" in english, but it's not a preposition nor a postposition. It's an adverb of location derived from the noun "sisä". To be precise, it's the allative case; just look at the declension: talo|oon (illative), talo|lle (allative), talo|ssa (inessive), talo|lta (ablative), talo|sta (elative), sisä|än (illative), sisä|lle (allative), sisä|ssä (inessive), sisä|ltä (ablative), sisä|stä (elative). The noun "sisä" basically means the inside of something, but it is not used (anymore) as a noun by itself. However, it appears in many compound terms like "sisäpuoli", "sisätyö", etc.. But, as an adverb it has many uses. Like, "Please, come on in." <-> "Käykää sisään, olkaa hyvä." Or, like in your example: "I went into the house" <-> "menin talon sisälle", which basically can have the same meaning as "menin taloon" or "menin sisälle", depending on the context. I'm not linguist, so my explanation can be a bit inaccurate. Hope this helps, though.