There are 4 ways to purchase you ticket:

In Cusco city via the ‘Ministerio de Cultura’ (The Ministry of Culture) It is located at Casa Garcilaso on Calle Garcilaso and the opening hours are Monday – Saturday 7am until 8pm. They except either cash, Debit or Credit cards.
In Aguas Calientes on Av. Pachacutec which is open everyday 5am until 10pm. This office ONLY accepts cash. Although location wise this is a convenient option please bare in mind that there is only limited numbers each day which can be allowed entrance. If they are fully book from pre orders then you may have to wait a few days. This is not a recommended option in peak season.
You can Purchase your tickets online ahead of time and potentially for a lower cost depending on how far in advance it is booked. The official government website is : https://www.machupicchu.gob.pe/inicio Things to note is that paying with anything but Visa could cause some problems or issues whilst booking. Make sure you take note of your reservation number as this is what is needed to access your tickets which shall be printable.
Through a reputable tour operator. This is most people’s first choice as they either do a trek attached to visiting Machu Picchu or do not want to organise the travel alongside the trip to the site. Tour operators also have set aside, allocated tickets so if they are booked out elsewhere a tour operator would be your best bet. Operators even on a one day visit will organise your train, vans/cars to get to the train and the bus up and down from Aguas Calientes. However the option is completely up to you and if you book your ticket in advance and then decide to join a trek or tour it would be an option to ask for this fee to be taken off your trip fee.
Please make sure you either have to hand, if ordering online or you have with you if purchasing in person : Your Passport, any valid card you are using for discount and the appropriate method of money.