Entrance
noun
A mouth that you enter is an entrance.
いりぐち
This word is really two separate words put together to form one, so let's look at it in two parts: The kanji 入 has り attached to it as okurigana (hiragana that comes after a kanji), so you can bet that 入り will use a kun'yomi reading for the kanji. You haven't learned this reading yet, so here's a mnemonic to help you: The entrance you're going through is really eerie (いり). It has cobwebs and creepy figures carved into it, and it just gives you a really eerie feeling. Since 入り uses the kun'yomi, you can guess that 口 will follow suit. You learned this reading with the vocabulary, but the く in くち changes to ぐ due to something called rendaku. Think of it as an eerie entrance into the Gucci (ぐち) store.
入り口は、どこ？
Where is the entrance?
入り口が三つあります。
There are three entrances.
まず、あなたのアパートの入り口のドアの大きさをおしえて。
First, tell me the size of the entryway door of your apartment.