Islam is the youngest of the three major monotheistic religions originating from the House of Abraham. One becomes a Muslim simply by bearing two testimonies, i.e., one declares that "there is no go but GOD (Allah in Arabic) and Muhammad is the Messenger of God." A Muslim need to believe in 6 fundamental beliefs (articles of faith) which are belief in (i)  God, (ii) His angels, (iii) His revealed books, (iv) His Messengers, (v) the afterlife and (v) predestination.

Besides the fundamental beliefs a Muslim is required to do five things, which are: (i) to bear witness that there is no god but GOD and the Muhammad is God's slave and Messenger; (ii) to establish regular prayers five times a day; (iii) to pay a prescribes amount of charity every year; (iv) to fast in the month of Ramadan; and (v) to make pilgrimage to Ka'ba in Makkah once in life time.

The links below will help you discover the above mentioned in detail.

1. Towards Understanding Islam by Abul A'la al-Mawdudi
http://www.al-islamforall.org/litre/englitre/Undislam.htm
This is a monograph in electronic format. An excellent introductory but comprehensive study in Islam originally written as a textbook for high school students. This book provides insight into Islamic beliefs. A good source for both Muslims as well as non-Muslims.

2. Introduction to Islam by  Dr. M. Hamidullah
http://www.muslim-canada.org/hamidullah_all.html
A very comprehensive introduction to Islam. Starting with a short biography of Prophet Muhammad's life the book describes a wide variety of topics related to Islam such as "preservation of original teachings of Islam," "faith and belief", socio-economic, political, moral and cultural systems of Islam, etc.

3. Islam: the easy way by Khurram Murad
http://masmn.org/Books/Khurram_Murad/Islam_The_Easy_Way/
An introductory writing explaining how simple and easy Islam is. Useful to understand the ways of  Islam for both Muslims and non-Muslims.

4. Guide to Islam for New Muslims by Sahibzada Hafiz Muhammad Abdul Wahid
http://www.irca.org.au/Islam/Introduction/guide01.htm
This guide details the basic beliefs, practices and sources of knowledge for new Muslims.

5. Introduction to the Articles and Pillars of Islam
http://www.usc.edu/dept/MSA/fundamentals/pillars/intropillars.html
Discusses topics like "the meaning of Islam," "articles of faith," and "Pillars of Islam and Application of Faith."

6. Let Us be Muslims by Sayyid Abul A'la al_Mawdudi
http://masmn.org/Books/Abul_Ala_Mawdudi/Let_Us_Be_Muslims/
A monograph in electronic format. Primarily targeted to people who are already in Islamic faith. Discusses the practical implications of the articles of Islamic faith such as Iman, Islam, salah -prayer, fasting, hajj-pilgrimage, zakat-obligatory charity, and jihad. Could be beneficial to those interested in Islam.

7. Principles of Islamic Faith (Al-`Aqidah Al-Wasitiyah) by Sheikh Al-Islam Ahmad Ibn Taimiyah
http://www.usc.edu/dept/MSA/introduction/wasiti/taimiyah_1.html
This e-book is an English version of thirteenth century Islamic theologian Ibn Taimiyyah. This is considered the most authentic of all writings about the creed of Ahl al-Sunnah wa al-Jama'ah (The Orthodox Branch of Islam)

8. Basic Tenets of Islam by Harun Yahya
http://www.harunyahya.com/tenets.php
Written by Turkish scholar Harun Yahya (Adnan Oktar) for those who are new to Islam or who have some conviction in Islam. Available in both acrobat and MS-Word format.