One can easily and profitably turn to the Wikipedia article on Sikhism, and use that as a gateway to other online resources. Here's a list of academically reputable resources, which might be available online if you have access via an academic library:
Singh, Pashaura. "Sikhism." Worldmark Encyclopedia of Religious Practices. Ed. Thomas Riggs. Vol. 1: Religions and Denominations. Detroit: Gale, 2006. 497-519. Online article. Paushaura Singh is Professor of Sikh and Punjabi Studies at UC Riverside.
Singh, Nikky-Guninder Kaur. "Sikhism." Encyclopedia of Religion. Ed. Lindsay Jones. 2nd ed. Vol. 12. Detroit: Macmillan Reference USA, 2005. 8393-8398. online article (for print edition, Hekman Library Theo Ref BL31 .E46 2005 ) Nikky-Guninder Kaur Singh is Professor of Religion at Colby College. Listen to her give an 18 minute audio tutorial on Sikhism's message, practices and identity in America on WBEZ radio.
Chilana, Rajwant Singh. International Bibliography of Sikh Studies. Dordrecht: Springer, 2005. Print copy - Hekman Lib Ref Z3208 .S42 C48 2005. A comprehensive bibliography of over 10,000 items in English, listed under 28 chapters and subheadings. The first chapter, "Printed and Electronic Resources," is an annotated survey of major reference documents and a selected listing of electronic and web-based resources.
Nesbitt, Eleanor. Sikhism: A Very Short Introduction. Oxford University Press, USA, 2005. Hekman Library BL2018 .N47 2005. A well-researched concise introduction with a helpful glossary and bibliography. From the Oxford Very Short Introductions series. See also Nesbitt's reflections on the methodological issues and challenges of writing such an introduction in "Issues in Writing 'Introductions' to 'Sikhism'," Religions of South Asia 1.1 (2007):47-63; doi:10.1558/rosa.v1i1.47