It is between the biggest three parliaments in the world. It is also nice inside. You can buy tickets on-site or on-line.
Parliament Parliament by night
Parliament – Interior Crown, Sword and Globus Cruciger
To Parliament the easiest way is to ride by metro.
You can take M3 metro line then you need to change to M2 metro line at Deák tér and ride to Kossuth tér station.

But other possibilities you have, too, that you can plan here: