well that's a broad answer really. if you're doing your own reading you could state the question out loud [i personally repeat the question quietly as a shuffle to focus on it, then state it aloud when i'm laying the cards] when it comes to other people it depends on the reader. some ask the person to say their question, i ask them to focus on the question while shuffling, but not to tell me [otherwise i worry i might misinterpret the cards] if this is an online reading however, i ask for the question so i may focus on the question as i shuffle [example: will username123 get the promotion at work?]
now, as for what questions are off limits, i don't really think there are any. sometimes being too specific might be a problem [will Timmy ask me out next Thursday, for example] it's also a little difficult to get a 'yes' or 'no' answer. certain cards can give a very positive or negative view of a question which could indicate a 'yes' or 'no' but few people actually do 'yes or no' readings. you also need to consider the future is always changing, so questions work better if it's about a picture [example: will i make honour roll this year?] a broad question will give a a spectrum of possibilities.
certain spreads also work better with certain questions, so if you're using a specific spread that gives you more information and possibility [i use a 13 card spread for example, it includes the best and worst case scenario and the two possible futures] then a question like 'will i be debt free within the year' would work as it could show the persons possible downfalls, hidden opportunities, and possible outcome if they stick to their current path. a three card spread on the other hand, is a lot better for something like 'does sue like me' [and you'll get love questions a lot, i've found a five card oracle spread gives the best answer] or more to the point 'i want a yes' questions.
you can also do one card pulls for simple things. many people do daily readings just to see what the energy of the day is like [much like reading a horoscope] i recommend trying this if you're new to tarot and want to get a feel for the different cards and their energies. you can spend the day reflecting on the card, carrying it with you, and trying to spend the day focusing on that specific energy.