should I look at the answers before I read the text? And then I start reading and choose the best one ? Or read the text and then look for the answer?
Hm, I don't think there is a single right answer to that question, it depends mostly on your personal preference and style.
When I do these and I read all the answers beforehand, I effectively have no memory of them by the time I get to the first blank. So usually, I'll start by reading the text, stopping at every blank and seeing if there's an extremely obvious answer that I can put in there. If I get lucky, then I can immediately eliminate some of the answers.
At the end, I'd then go back to the options where I couldn't immediately figure out what to put, and I'll check which of the remaining options makes the most sense. (Keeping in mind that there are more answers than blanks, so not all answers will be used.) But you might find that a different strategy works better for you. Ultimately, you'll end up reading both of them many times as already said.
my second question is that the text before the blank number is important or the after one or both of them?
It depends on the sentence, but since you need to put a sentence into context, the context can be found before or after the sentence, or both. If you think about it, a sentence could relate to a previous one - "My mom took me to school. The car broke down.", it could provide information that the next one expects - "We had cake today. It was delicious.", or it could also just fit between two other sentences and they both relate to it - "Bob gave me a toaster. We used it to make some toast. Bill choked on a piece and died.".