Tomorrow I am going to 北京。 you can say： 我明天要去北京 or put "when" at the beginning of the sentence 明天，我要去北京 要 in the sentence means the future tense.

应该说：”明天我要去北京“或者”我明天要去北京“。 ”想“也可以：”明天我想去北京“或者”我明天想去北京“ Just a note, I'm also still in the process of learning Chinese, but maybe that makes it easier for me to answer your question. I have learned to put Chinese verbs such as 想，要，去 always together (in one group) in a sentence. I hope this helps!

明天我要（It think 想 is better here) 去北京。It's like in English "I want to go to (place)."

-->我明天要去北京

明天我要去北京 or 明天我要到北京去。