performance tuning queries - practice
You can begin from reading on MSDN Query Tuning.

Initially to Begin with , find the Queries which needs to be tuned on basis of how bad they perform in terms of resource consumption or time they are taking to execute and then analyse their execution plans accordingly.

There are many free tools available out there apart from SQL server inbuilt tools like DTA etc, which makes the analysis go easy.

The best that I feel comfortable while doing query tuning is using SQL Sentry Plan Explorer.

You can use SQL Sentry’s Plan Explorer to get faster performance and tuning information from SQL Server query plans.

Also please read this excellent article Finding the causes of poor performance in SQL Server

Initially above should get you started. Apart from that you can always refer to SQL server Books Online