(mija learning Spanish from day one)
One of the best things you can do if you plan on teaching your child a second language (in our case, Spanish), is to teach it from the start along with the first language. That is, teach both simultaneously! While it's certainly possible to teach a child to speak a second language to native levels after birth, such as through dual language education programs, the process is much easier if you can start the target language from day one, just as you would if you were raising the child monolingually.
When you start teaching the second language, or L2, from day one, you give your child the gift of time. Think about it: Why do you frequently hear adults struggling to speak foreign languages when there are so many preschoolers all over the country who seem to speak said foreign language fluently? In most cases, it's because those preschoolers have already spent much more time absorbed in the language than have the adults.
If you spend five years hearing Spanish and nothing but Spanish during your waking hours, and are constantly spoken to in Spanish and responded to and reinforced whenever you make an effort to speak Spanish, and simply need to be able to communicate like a five year old at the end of the period, you'd probably get pretty good at Spanish too! Especially if you didn't have any adult or family responsibilities, and simply got to live like a kid with lots of attention and playtime. And you'd have an advantage over a 5 year old, because you can read books!
In other words, if you want to learn an L2 as an adult, you need to do whatever you can to get as much input in that language as possible within the confines of your daily life. And if you want to teach an L2 to a child, then you need to give that child as much input as possible, as early as possible, and for as long as possible. A 14 year old raised bilingually from birth will have a 14 year head start on someone who starts taking Spanish for the first time as a 14 year old in high school. And while it's definitely possible to make up for lost time if you increase your rate of input, it's much easier if you don't have to make up that time to begin with.
Start early. As early as possible. And don't stop.