It's impossible to make a budget without a very clear understanding of the costs associated with your chosen destination. Everything needs to be accounted for, from flights and accommodations to meals and ticket prices for attractions. Make a general itinerary and dive into online resources such as travel blogs and official tourism websites, as well as published travel guides (get the most recent!) from a library or bookstore.
Watch out for seasonal fluctuations; costs and crowd sizes can vary widely from high season to shoulder season to off-peak times, based on weather, local religious observances, and a variety of other factors. Remember that there might also be expenses before jetting off, such as passport fees, travel insurance, pet care, and phone service. Upon arrival, is it cheaper to rent a car or feasible to use public transit? If you're planning to go abroad, what's the currency exchange rate? What are the tipping customs? It may seem overwhelming, but that's why it's key to start early.