The TOEFL is preferred by many American universities (though many also accept the IELTS, and some may only accept it. Be sure to check with the universities you are most interested in.) It is accepted by over 9,000 universities in well over 100 countries. Its website explains the test and has information about where it is given, how to register, and how to prepare.
It offers a lot of free practice and preparation tips, including several “Inside the TOEFL test” videos on the skills tested– and for the writing test even 3 videos on the questions you will need to answer. (It also gives sample questions and old practice tests. To find them, click “Prepare for the Test” and scroll to near the bottom. The free practice materials are below materials with a price.)
You can also pay for extra help: online practice tests quite similar to their actual test, a guidebook, a 6-month subscription to an English reading improvement service, or other options.
As their reading materials point out, the best way to improve your reading skill in English is to read. Read all kinds of materials in English, and read often. You don’t have to pay for such a service to prepare for the TOEFL. However, you might look at it if you need help staying motivated, or if you have trouble finding materials easy enough to understand. (For reading selections, also try Online Reading and the list of readings they suggest.