It is often confusing to use proper particle when starting a sentence in Japanese. Please watch the video below and understand one way to distinguish the difference between は and が.
One more function of が that you need to be aware of is "however," and "but" function. When が is placed at the end of the first predicate, it adds "although," "though," "however," or "but" meaning. For example:
わたしは アメリカじん ですが、にほんに すんでいます。Although I'm an American, I live in Japan.
えいごの せんせいは きびしいですが、クラスは たのしいです。Although English teacher is strict, her/his class is fun.
Simplified explanation (means there are many, many other ways of は and が)of knowing the difference between the particles は and が for beginner level in Japanese language.
For additional particle practice, go to the link below and fill in the blanks with different particles. By Texas A&M.
http://laits.utexas.edu/japanese/joshu/grammar/gparticle/gpquiz/gpq_level1.php