When you treat an alkyl halide with a nucleophile/base in a certain solvent, what's the likely reaction?  The flow chart below is designed to bring it all together for you, conceptually.  If you plan to jot it down (or, more likely, print it), scurry into a corner, and commit it to memory, please don't - it can't be done easily, and you'll just get very confused, demoralized, and thinking that a Leisure Studies major sounds pretty good about now.

The flow sheet below is meant to be used from left to right.  That is, the first step is to classify the halide structure (are unimolecular or bimolecular reactions more likely?) Then, assess the nucleophilicity vs. basicity of the nucleophile (is substitution (more Nu:) or elimination (more basic) likely to predominate?)  And so on....

Just remember the following:
- Nucleophiles are basic and bases are nucleophilic, but can be classified as more nucleophilic than basic, or visa versa.  [Hint: HO^-, RO^-, and NH₂^- are strongly basic; the others aren't.]
- Solvents can be classified as protic (having an OH group) or aprotic (not having one).