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Use white space and line breaks to make your query easier to read.
Specify the fields in your SELECT. Don’t just rely on SELECT * to pull back every column in the table.
Specify your JOIN types. INNER JOIN and LEFT OUTER are the two most common starting points.
Specify the conditions of your JOIN with the ON statement. How are your two tables related to each other? Don’t make us guess.
Alias your tables to make your query easier to write and read. Aliases can be very short or descriptive.
Learn about SET XACT_ABORT and whether you need it for your SP to behave the way you think it should
Define good datatypes (not over sized for no reason)
Make sure parameters and variables you are comparing against columns match datatypes.
Functions can kill your query performance, try not to rely on them. If you do, use them outside of/prior to your big queries.

SELECT A.col1, A.col2, A.col3, B.col1, B.col2, B.col3
FROM TableA AS A
INNER JOIN TableB AS B
ON A.someID = B.someID;

Most RDBMS come with some form of sample database. Here is the documentation for Microsoft SQL Server