Both に and は are very common Japanese particles, with completely different purposes.
- に ("ni") is a place marker, like English 'in' or 'at'.
- は ("wa") is a topic marker. It's somewhat similar to English 'speaking of ～' or 'regarding ～'.
- に and は can be combined, and the meaning is just the combination of the two. It's translated as "In ～, ..." at the beginning of the sentence.
I speak English fast.
Perhaps you have misheard something. You don't need the place marker に ("ni") to say this, and I have no idea how I can use に. You only need the topic marker は, which replaces を ("o"), the object marker. (Read "Object as Topic" section in this page if you are not sure)
えいごは はやく はなします。
Eigo wa hayaku hanashimasu.
(lit. "As for English, [I] speak [it] fast.")
I have 7 people in my family.
Your "Japanese" sentence has a lot of spelling errors, but did you want to say the following?
かぞく に は しちにん います。
Kazoku ni wa shichi-nin imasu.
(lit. "In my family, seven (people) exist.")
Okay, this is not very natural, but at least grammatical and understandable. Here, かぞく is marked with both the place marker and the topic marker simultaneously. に is used (but actually not required in this case) because you're talking about who is in your family. は is required because かぞく is the topic of the whole sentence.
My children are in America.
わたし の こども は アメリカ に います。
watashi no kodomo wa america ni imasu.
(lit. "As for my children, [they] exist in America.")
Again I corrected the obvious spelling errors. In this case, アメリカ is marked with only に, because the topic of this sentence is something else.