1- It is not a sin to stay unmarried for the rest of your life as long as you are sure that you won’t fall into sins because of being single.
2- Yes it is recommended to get married. It’s the Sunnah of our Prophet (s). If someone good and righteous comes your path, do take the opportunity. But it’s not haram if you don’t want to.
3- If you do get married, you are not obligated to have children. It’s not a sin to not want to have children. So that’s fine. Yes, it’s recommended to have children.
You may see this video about that: