This little section of UofS's site has a nice, friendly account of theme-rheme structures and other features of cohesion, as well as useful guides about how to write and structure a general academic essay. I've linked to the section on cohesion, but look also at the tabs on grammar, and how to cite sources.
(Be aware that your writing for A Level may not follow exactly this style -- check the requirements of each module. But it's good general advice!)
2. Strategies for creating cohesion - Page 1
