America is known as a melting pot... and for good reason. People from all around the world move to here and bring their culture and customs as well as their native languages with them. Parents who speak English as well as languages other than English can sometimes be faced with the dilemma of whether or not to pick only one language to speak at home. Then, if they decide to use only one language, they must decide which language to use.
If you are in a bilingual household and both languages are spoken to your child, it may take a bit longer for your child to develop language skills in both languages. For example, your child may begin to speak a little later than expected, or may babble for a longer amount of time than children learning only one language. Also, while your child is still a toddler, it is not unusual for bilingual children to mix both languages in one sentence when speaking. Eventually as they get a little older (around 4 years old) they work out the kinks and begin to use each language separately.
If you speak only English in your home, but would like your child to have exposure or start learning a second language, around the age of 3 is a good age to introduce a new language to your child. This is because young children absorb language without even trying.
If you want to introduce your child to another language here are some good resources:
Usborne publishes books in languages including English, German, French, and Spanish:
My personal favorites are the "First Hundred Words" books which have great pictures and words in English and the featured foreign language.
Janelle publishing has picture cards and many more materials in Spanish:
Uncommon goods sells fun flashcards in Spanish & French:
Put on shows like Dora, Go Diego Go and Handy Manny which are mainly in English, but mix in Spanish as well.
Looking at picture cards and books with foreign languages in them are also great ways for adults to learn another language too, so it's a fun way for you to learn with your child.
