The question is much more complex than you might think on the first sight. First of all you have to think about the theory you want to use. For example generative grammar or SFL say completely different things about L1 language acquisition.
Before you can talk about language acquisition you have to understand the way of how language works in general and thus you should look at the theories. I personally like SFL although my studies in psycholinguistics slightly changed my opinion. Nonetheless, SFL is still one of the best theories for beginners.
It is some time ago that I worked with language acquisition but I will try to give you an idea of how it looks like.
A baby normally starts to learn the phonemes before actually being able to use them. Language acquisition is highly dependent on input. After some time the children then start to use them to train their brain and vocal tracts. The first part (roughly 0-6 months) is the time in which the baby mainly receives information. It will also use some of the sounds it heard and make pointing gestures together with them in order to show what it wants, but the sounds do not have a clear meaning at this stage.
In the next stage (6-12 months) the baby will start to understand some utterances in the context and will also start with the babbling phase. From there on I can not really remember the months etc. But basically the child will go on with one word utterances then two word utterances and then after some time with phrases. The more input the child recevied the more it learns and the faster it can produce language in a appropiate way.
If you search for L1 Acquisition you should find many studies and papers about that topic. Just keep in mind that different theories and departments think differently about this.