To practice Tibetan Buddhism, you need a qualified teacher. In Tibetan Buddhism there are two things which as sutra and tantra. At most, you can pretty much study sutra by yourself through books (but then understanding the real meaning is a whole different thing, which is why we still need a teacher for sutra) AND tantra, you can only practice once you receive initiation from the teacher.
For example, I'm from the Tibetan Buddhist tradition and my teacher is H.E. the 25th Tsem Rinpoche.
Why do you need a teacher? Because like everything else, we needed a teacher in school, so we also need a teacher for spiritual school.
I would also recommend books or websites that you can learn from. Some books that I like reading are Geshe Sonam Rinchen's books, Geshe Kelsang's books and Alan Wallace. You can find them from Wisdom Publications. Alternatively, you can listen to dharma talks on Youtube. I'm assuming your main language is English, so search for Ajahn Brahm or Tsem Rinpoche, both teachers speak very good english. Ajahn Brahm is Theravadan, but his talks are very nice to listen to as he integrate Dharma into daily life. You can also visit my teacher's blog
www.tsemrinpoche.com to read up about Buddhism there.
HOWEVER, for practice, I would highly recommend you find a teacher and dharma center near you. That is the MOST effective way to practice Tibetan Buddhism. While you search, along the way you should also read up the qualities of a spiritual guide and of a student. Some books you can read to widen your knowledge and understanding on this subject (i've read all 3 of them):
Wise teacher, wise student by Alexander berzin
Dangerous friend by Rig'dzin Dorje
Gurus for hire by Tsem Rinpoche
Its important for us to check if the teacher we are going to learn the Dharma from is genuine. It's just like when we are buying a car or a house, we don't just purchase the first car or house we see... we research, make sure it is suitable for us, etc. So just like that, the teacher who will lead you to Enlightenment and will be giving you the most precious thing in life which is the Dharma... you need to check that it comes from an unbroken and pure lineage that goes all the way back to Shakyamuni.
Good Luck!