If you're starting from scratch, I recommend finding either a Latin class at a local university or to contact a tutor to help you; learning a language on your own is extremely difficult, as there are a lot of idiosyncrasies that are difficult to grasp without assistance.
That said, if you're looking to start on your own, you should find either a textbook (I highly recommend Wheelock's Latin) or a free video class (I don't know any of them, but you should be able to find one online). From there, you'll learn the many verb and noun forms and the most common grammatical constructions, before moving onto reading real Latin texts.
Translating Latin is what you probably are going to want to do with Latin, at least at the start. Julius Caesar's Gallic Wars is usually considered a good introductory text, as is any of the biographies by Cornelius Nepos, but you should read what you want to read; otherwise, you won't be motivated to actually do the work, and your motivation is the only thing keeping you honest when you are studying on your own.
As for writing...composing Latin is hard. Just like reading, what you need to do is practice it. Find something you want to translate into Latin, then translate it into Latin. And repeat this a lot, because as I said, Latin composition is hard; I have studied Latin for almost ten years now, and it takes me a lot of time to compose even one line.
