Here's an interesting formula to think through your question:
In a democracy, anything that is not forbidden is permitted.
In an authoritarian system, anything that is not permitted is forbidden.
In a totalitarian regime, anything that is not forbidden is mandatory.
Now, to get the facts:
An autocratic government means that political power is concentrated in the hands of one person, whose decisions are subject to neither external legal restraints nor regularized mechanisms of popular control. In a military dictatorship, political power is held by a military junta, militia or other form of armed authority which, again, is not subject to restraints or effectively accountable to the public in any way. By contrast, a democratic government is elected by the people, supposed to represent the people and accountable to them.Both an autocratic regime and a military disctatoship could be (and often are) totalitarian regimes, in which the state strives to control every aspect of life and civil society; or authoritarian regimes which offer citizen very limited freedom. What makes both of them significantly different from a democracy are the absence of free and fair elections and the absence of accountability to the public.
Now, as to how an autocratic and a military dictatorship are different from each other, the main aspect you need to examine consists of power relations within the governing body. In an autocratic regime, everyone is bound to obey, under threat of severe penalties, the supreme ruler (absolute monarch, president, Glorious Leader etc.). Nicolae Ceausescu's totalitarian regime in Romania was a good example of an autocratic totalitarian regime. In a military dictatorship, on the other hand, political power resides with an armed military group rather than one person; which may be organised internally more or less hierarchically or democratically; nonetheless, the governance reflects the political will of the group as a whole rather than of the supreme leader. A lot of military dictatorships in Asia and Latin America are ruled by a military junta, a committee composed of several officers, often from the military's most senior leadership. (Burma is a good example to use).
It is important to note that a military dictatorship can also be an autocracy, although not all autocracies are military and not all military dictatorships are autocracies. The overlap can happen when governments are led by a single powerful military leader, or by a military regime with a highly hierarchical structure, with only one person at the top. Gaddafi's regime is an example.
In order to approach your assignment, with these definitions in mind, you could look at the structure of government in the countries I have mentioned and compare them against each other, as well as against democratic countries