1. Remember the earlier, the better. By the age of two, a child's brain is sucking up and processing all the information he or she comes in contact with, so it's an ideal time to start learning a second language. You may think, "But he hasn't even mastered ENGLISH yet!" That's okay! Believe it or not, children can easily juggle two (or more) languages at once without the difficulty that their adult counterparts may have if they try to learn the same language when they're older. Learning two separate languages will also not hinder your child's reading or speaking ability. In fact, it will actually improve and broaden their vocabulary as they get older, since many foreign language words are used in English as well.
2. It is better to teach your child with some simple words and basic greetings first. Learning a language does take time and repetition, so only move on when your child feels comfortable and confident in using what he or she has already learned. If you're not completely fluent yourself, but still want your child to have the benefit of a second language, purchase a good dictionary and work book to reinforce what you already know so that you can teach it to your child.
3. You can try to make studying language to be a fun time by setting aside a special "Language-Only" time. Practice the language together with your child through music, books or videos and use this time to speak and play together using only the second language. It's a great way to build up the child's interest in another culture and will help him learn the language more completely and thoroughly.
4. If your significant other speaks a foreign language, consider having him or her speak only in the foreign language to the child, while you speak to them in English. This will quickly help the child understand each language's structure and differences, and he won't be as likely to get confused. Remember though, language learning is a slow by steady process. Let your child know not to worry about making a mistake or sounding silly. That's the only way he or she will become confident enough to speak the language on a regular basis.
5. Whenever possible, immerse your child in the second language as often as you can. Taking trips to the place where the language is spoken is a great way to make a language feel "real" to a child. Children mimic what they see, so seeing others speak and use the language in their day-to-day lives will show him that he can do the same!
The most important thing of all is to treat learning a second language like a fun "adventure" rather than a tedious chore. If you keep these tips in mind, it won't be long before your child is equally comfortable and proficient in both languages! Good luck!