"The important thing about demonstratives is that we use "isso, esse, essa" much more than "isto, este,esta" , and the difference between them, in colloquial language, is none.

"Esse menino é um chato "or "este menino é um chato" are both absolutely correct in spoken informal language, but if your being tested at school, for instance, then you'll have to keep to the idea that "este, esta, isto" refer to a person or object near you, while "esse, essa, isso" are generally a little farther, out of reach. I myself use isso, esse, essa all the time.

It's very common to add "aqui, ali,aí, lá" to these pronouns, such as in:
"Eu prefiro esse aqui" = "Eu prefiro esse"
or,
"Quem é aquela moça lá?" = "Quem é aquela moça?"
"Qual moça? - "Aquela lá"
without any special meaning, just emphatic.

Aquela, aquele, aquilo, they take the ` , the crase. "Eu me refiro àquela senhora ali." The demonstrative aquele, aquela, aquilo, is the only word taking crase besides the article a.