"I am going to Beijing next month. I need to buy the flight ticket now."
By the way, in this case 得 should pronounce as dei in stead of de. Dei 得 means you have to.
要 also implies future tense. eg:要下雨了。(It is going to rain.)
In this case, the sentence is also correct without 要, but I feel that it's more natural with it.
It kinda works tho in this sentence. Like you're going to, or you want to. I feel that Duolingo tends to emphasize the word like this. Like 我要去上学。which means I'm going to school.