This link seems to be a very good source with respect to Islamic theology.
This link as well is very good for the basics of Islamic theology.
From my understanding, the theology of Islam is not too in-depth as say Judaism or Catholicism. Unlike Judaism or Catholicism, in Islam one is not tested over a several month long period in front of religious authorities before being able to convert. In the process of converting to Islam, depending on the Islamic sect, one must simply affirmatively declare in Arabic that there is only one God Allah and that the Prophet Muhammad (s.a.w.) is His Messenger in front of at least two Muslim male witnesses. These two basic principles of there being one God and the Prophet (s.a.w.) as His Messenger form the theological core of Islam.