It is not the easiest starting point. Demons is long, about 880 pages in a standard edition, with many characters bearing Russian names and nicknames, and a slow First Part, a comedy of manners. Those looking for early Dostoevsky are usually better served by White Nights, Notes from Underground, or Crime and Punishment.
Against it, beyond its size: the book opens slowly on purpose, as the portrait of an old liberal and a hysterical friendship, not as a political novel, and only in Part Three does the disaster arrive in full. The narrator is a chronicler who is ironic and reconstructs from other people's memories, and the Russia of the 1860s lies behind every sentence.
In its favor: it is the author's most directly political novel, and what it shows remains valid outside the Russia of that time, because the group does not win by force; it wins because the town does not want to seem backward. It has unforgettable scenes, such as the night Stavrogin visits Kirillov and Shatov, the meeting at which Shigalyov reads his system, and Kirillov's death.
A tip: read it in order, without skipping the First Part, because its slowness is what makes the Third work, and decide where to read "With Tikhon": after Chapter VIII of the Second Part, where the author wanted it, or at the end, as the editions print it. Anyone who prefers a lighter pace can follow the reading schedule, of about 10 pages a day.