How do I start a compare/contrast essay when the subject is to compare the Spanish and British and how they colonized?

You could start off with some context, something general about how the European nations became involved in colonization on a large scale starting from the 1400's. Then bring Spain and Britain into the picture, naming these two countries as two major colonizers. Then you should go right into your thesis statement, which should clearly state the main aspects of British and Spanish colonization that made them similar and the main aspects that made them different. I would mention the following:

Similarities

1. Both set up plantation systems for economic gain: the Spanish in the Carribean and South America, the British in the Caribbean

2. Both were major colonizers that subjugated many peoples and conquered large areas

Differences

1. Time period: Spain was one of the earliest colonizers, while Britian was a little late to the scene. In fact, a naval battle between Spain and Britain in the 1500's marked the end of Spain's heyday as a major colonizer and the beginning of Britain's golden age.

2. Though British colonizers also sent up a plantation system on the Caribbean islands, the system that they set up in the North American mainland was quite different. British colonists on the mainland enjoyed a fair degree of autonomy as farmers, merchants, or craftsmen. 

3. Assimilation with Native Americans: Spanish interaction with the Native Americans they conquered was far greater than British interaction with Native Americans. The Spanish wed and had children with the Native Americans, and introduced many elements of their culture into Native American culture. The British, on the other hand, did not mix nearly as well with the Native Americans, and the British colonists and the Native Americans remained segregated from one another and hostile to one another.