Dostoevsky is not hard to understand, but he demands patience: the plots are strong, the conversations are long, and the novels ask for attention to names and ideas. For someone who has never read him, the best starting point is Crime and Punishment.
What complicates things are the Russian names, with given name, patronymic, and nickname (Raskolnikov is Rodion Romanovich to almost everyone and Rodya to his family), the monologues, such as that of the drunkard Marmeladov in the tavern, and the ideas, which come on stage in the mouths of characters who think out loud. It helps that the novels were published in serial form: each installment closes on a point of tension, and the reader wants the next one.
One possible order: Crime and Punishment, which lays out the whole problem in its first seven chapters and has an examining magistrate as its adversary; then The Idiot or Notes from Underground, a short, harsh novella that is drier reading. Demons and The Brothers Karamazov are for later, because they are long, more political and theological, and reward readers who already know the author's method.
Two tips: use a translation made directly from the Russian, because the books depend on keywords that get lost in a secondhand version, and do not try to hold all the names in mind at once, because the main characters repeat a lot. Some find Dostoevsky difficult and others find him addictive. The text supports both reactions, because the same traits, the excess, the fever, and the argument, tire some readers and grip others.
On Clavis: Crime and Punishment.