We use a full stop at the end of a sentence. Usually, each sentence has one idea.
That is a good book. I read it last month.
There are two sentences in the example: 'That is a good book.' and 'I read it last month.'
Notice the full stop after each sentence.
Full stops are common. We use them a lot in English.
When you use a punctuation mark, you need to put a space after it. Don't use a space before the punctuation mark.