Welcome to the site.
I would start off looking at wiki.
Then, the faq book on theosophy - Key to Theosophy by H P Blavatsky. This can be found free on Internet. Search Google.
Blavatsky was solely responsible for the introducing theosophy to the modern world.
Since no one's word is final as far as understanding of theosophy, you are on your own to read other materials and come to your own understanding of the universe and human being.
From what I have seen, exposure to theosophical ideas gives a direction in day to day life and has a transformative effect.
Feel free to ask any question and many would be willing to respond.
I would like to introduce you to one of the greatest finnish theosophical writers, Pekka Ervast
From this link you will find the complete list of free pdf books online from this author. There are couple books also in english but most of them are in finnish or swedish.
And here I'll post couple links straight to english books if someone finds it difficult to navigate on a finnish page.
I hope you enjoy reading these!