The Secret Doctrine is - still after a century - one of the most popular books in the theosophical literature. Yet everybody who tries reading it for the first time has issues. Luckily there's a document that helps newbies wade their way through it:
The "Secret Doctrine" and its Study
Being extracts from the notes of personal teachings given by H.P. Blavatsky. to private pupils during the years 1888 to 1891, included in a large MSS volume left to me by my father, who was one of the pupils. -- P.G.B. Bowen
Of course that is still only a start.
I personally also recommend the so called third volume of the Secret Doctrine - most of it anyhow. I read it before I dived into the Secret Doctrine myself - it has lives of initiates and all kinds of other things that help you get into the theosophical mindset as it were. Most of the content in there is also in
Volume 12 of the Blavatsky Collected Writings - which is also well worth reading. Those new to theosophy should ignore the Esoteric Instructions that are also contained in both volumes, those really are too difficult to understand. They require that one has a basic understanding of The Secret Doctrine itself.