Firstly you need to know what totalitarianism is
Some of the things you should look at/research:
- Cult of Personality surrounding Stalin
- Secret Police (OGPU, NKVD)
- Work camps
- Political prisoners (especially cultural workers)
- "Kulaks" and "sub-Kulaks"
- Government's complete control over the press/media (cinemas, filmmaking, commissioning of certain films, censorship, ownership of the press etc etc)
- Government's control of cultural workers
- Purges and show trials (1937/8) of the Communist Party and the Red Army
- How the Communist Party was set up: Stalin had complete control and power (how/why)
- The 1936 Soviet Constitution (which basically said "err yes... we have democracy" but one of the clauses in it totally invalidated anything of the sort lol)
- School textbooks which presented a pro Marxist-Leninist view of history which elevated Stalin to a godlike status
That's all that I can think of off the top of my head, I'm not going to go into detail because you will remember the information related to these better if you research it yourself.
http://www.wikipedia.org is a very good place to start researching (and at times can be the be all and end all of your investigations!).
You should also know what assess means in regards to the HSC (make a judgement of value, quality, size, outcomes etc etc)...
You can structure it either chronologically or the preferred method which is by concepts. So control of the press and indoctrination in schools etc etc can be grouped together with political (purges, persecution of kulaks etc) grouped together as well and how all of these events/issues collaborate in your judgement of the USSR under Stalin being a totalitarian regime. Don't forget to define totalitarianism (at least to some extent) in the introduction!
Happy hunting