The Upanishads are  the philosophical foundations of Hinduism - profound yet practical philosophies about reality, consciousness, and spiritual liberation.From ancient texts, in most cases, one should always look for what has something new to say to understand as well as assimilate the Hindu form of wisdom. 
 
Here is a very short survey of some of the key Upanishads and their central ideas:
 
1. Brihadaranyaka Upanishad 
 
The Theme: Nature of reality and self Key Message: This is the oldest and also longest Upanishad. It deals with the relationship of Brahman (the ultimate reality) and Atman (the individual soul). Notable Verse-: Aham Brahmasmi (I am Brahman)-1.4.10. This mahavakya (great statement) declares the ultimate identity between self and cosmic reality. 
 
2. Chandogya Upanishad 
 
The Theme: Nature of consciousness and meditation Key Message: It tells how the universe and consciousness are interconnected through allegories and dialogs. Notable Verse: Tat Tvam Asi (That Thou Art)-6.8.7. This great utterance, repeated nine times, avouches that our essence is that of the divine ground of being.
 
3. Isha Upanishad
 
Theme: Balanced living through renunciation and action Key Message: Teaches how to live in the world while holding firm to spiritual consciousness. Notable Verse: Tena tyaktena bhuñjīthā (Enjoy through renunciation) - Verse 1 - Encourages enjoying life's gifts without attachment, seeing the divine presence in all.  
 
4. Katha Upanishad 
 
The Theme: Knowledge of the self, reality, and death Key Message: Its exposition deals with the understanding of death through a dialogue between young Nachiketa and Yama (Death). Notable Verse: The Self is smaller than the smallest, greater than the greatest"-2.20. The paradoxical name describes Atman transcending all limitations. 
 
5. Kena Upanishad 
 
The Theme: The source of knowledge and consciousness Key Message: Analyzes what leads awareness into being and limitations of intellectual comprehension. Notable Verse: That which speech cannot express, but that whereby speech is expressed... That alone is Brahman-1.4-5. Speaks to the transcendence of ultimate reality as beyond conceptual understanding. 

6. Mundaka Upanishad  
 
Higher and lower knowledge Theme: It talks about distinguishing intellectual knowledge from change-producing spiritual knowledge. Notable Verse: "Two birds sit on the same tree one eats the fruit while the other watches" - 3.1.1-2 Symbolizes the empirical self (experiencing pleasure and pain) and the witnessing consciousness.
 
7. Mandukya Upanishad 
 
The Theme: It deals with the nature of consciousness and the holy syllable OM Key Message: Analyses the four states of consciousness through the symbolism of OM. Notable Verse: Ayam Atma Brahma (This Self is Brahman)-1.2 Declares the identity of Atman with Brahman, yet another mahavakya.

8. Taittiriya Upanishad 
 
Theme: Covers layers of human existence and bliss Key Message: Talks about the five sheaths (koshas) that conceal the true Self and explains the nature of joy. Notable Verse: Satyam vada, dharmam chara (Speak truth, follow dharma) - 1.11.1 Foundation of ethical principles for the spiritual effort.
 
9. Aitareya Upanishad 
 
Theme: Creation and the universe consciousness Key message: It speaks of how consciousness comes about by creation. Notable Verse: Prajnanam Brahma (Consciousness is Brahman) - 3.3 A mahavakya that identifies pure awareness as the ultimate reality.   

10. Shvetashvatara Upanishad 
 
Theme: Personal divine and yoga Key Message: Introduces theistic elements and early notions of yoga. Notable Verse: God is that one who spins the web of this universe, who makes all and beholds all- 6.1 Presents a more personal conception of divinity.
 
11. Prashna Upanishad 
 
Theme: Creation, life-energy, and meditation Key Message: Explores important aspects of existence through six queries. Notable Verse: Those who meditate on AUM attain Brahman-5.2. Links meditation with spiritual realization. 

Central Teachings for Beginners
 
There are the most salient truths from Hinduism that are Indian to the newcomers-one or two of such truths being:
 
Unity of Existence: All reality interconnected as emerging from one source (Brahman).  
 
Divine Nature: Your true self (Atman) is not separate from the cosmic consciousness.  
 
Direct Experience: Spiritual truth must be realized, not merely intellectually understood.  
 
Ethical Foundation: Truth, non-violence, and compassion form the basis for spiritual growth.  
 
Meditation: Habitually cultivating contemplative practices reveals inner wisdom that transcends the thinking mind  

It's not that the Upanishads require blind faith; they go well with personal investigation through self-inquiry and meditation. They are almost prescient in their relevance, being constructed as frameworks for finding purpose and meaning that cut across cultures. 
 
Also start from the shorter texts (Isha, Katha, Kena) before you proceed to the larger texts. The wisdom that is encoded in these old texts still brightens the way of spiritual seekers across the world, irrespective of their backgrounds or what beliefs they came with.