これ + は / が / を / も / の
これ means "this" or "this one": a thing near you, the speaker. It is a noun, so は and the other particles follow it directly, and it never has a noun after it.
It is one of a set of four:
Japanese points with three words where English has two, and the word changes with the speaker. What is これ to you is それ to the person facing you, so a question with これ is often answered with それ.
これ stands alone. To say "this book", with a noun following, the word is この.
Mixing the two is the usual mistake: ×これ本は面白いです and ×このは面白いです are both wrong.
を, も and が follow これ the same way は does.
These go beyond pointing at a thing you can see.
これ is for things. Said of a person standing next to you it sounds rude, so say このひとこの人, or こちら when you introduce them. A person in a photo can be これ.