SQL Server is multi-cpu and multi-core capable and will use these if certain conditions are met. What are you trying to do that makes you think you have to consider multi-threading? SQL Server takes care of those things and it is rarely a point to worry about in the beginning, Reads/Writes is something that will cause issues well before CPU/Multi-threading in most cases.
SQL Server is a multithreaded application. as @Fatherjack said it decides at execution time whether to use multiple threads on a single query. For overall work SQL Server will roughly balance its load across all processors unless you specify otherwise or are running in certain NUMA situations. SQLOS has a built-in thread manager that maintains a stack for waiting processes. Having said all that it's mostly out of your control; suffice to say that you can launch a lot at it and SQL Server will run as much as it can concurrently.