When you embark on a language learning journey, you are taking on a long and enduring effort. So therefor, it is best to choose your learning materials wisely. I would like to give you some insight on what to consider when you plan your route.
First and foremost, for the beginner there are a myriad of language courses you can buy. Beware of the claims made by these courses. They do not take you to a fluent level in the language. They do not even take you to an intermediate level. When you choose one though, you should try to find a comprehensive course that covers all of the grammar constructs in the language. Verb conjugations, lots of vocabulary, and phrases should all be covered. Some people make the mistake of thinking that because they know all the grammar rules that they are at an advanced level. The grammar rules, however, are just part of the basic level. There is a lot of grammar to learn which does not involve any rules and you have to learn these things one by one as you come across them. That is the intermediate level.
It is important to go through all of the language learning material in one series. If the series you are thinking about using does not go as far as another series, you should use the other series. The danger of having to switch series lies in the fact that the new series you start using may have covered some things in the earlier volumes that you never had in your other materials, and so you will miss that entirely. If you have to restart with a different series, you will be bored studying material that you already know, not to mention wasting time. That is why you want to do all of your book-learning from start to finish and not having missed anything or needing to find something that picks up where the other left off.
After you have learned everything you possibly can from textbooks or language courses, you will be at about the intermediate stage. From here, you can begin using materials written for native speakers, such as novels, newspapers, and other published materials. You will need lots of input from these sources to get up to the advanced level in the language.
How do I know the importance of completing an entire course in the language you are trying to learn? Because I wasn't able to. I took Japanese classes for 2 years and used a very good textbook. There were at least 3 volumes to this textbook and we were able to get about half-way through the second volume. So I did not learn all of the grammar that could have been taught while I was taking those courses. But obviously, I did not need to start from the beginning. Yet I couldn't demonstrate how much I knew. So when I came to Japan and went to a Japanese classroom taught once a week by volunteers, they didn't know what to teach me. I was taught like a very beginner about things I already knew. But I couldn't join a higher group because I wasn't that far along. So obviously it just became a waste of time. You can learn more on your own just by finding and tackling the areas you need to work on. Finding them is not easy, though. That's why it's important to work through a language course from the beginning until as far as possible. Then you won't be left with any gaps.
Don't be afraid to use more than one series. If you have the time, learning the same thing again is not all bad. If it does not bore you, you can strengthen what you already know and learn other things better. It might be best to use this approach from the very beginning while the subjects are still rather new to you.
I've just briefly introduced my ideas here, so if you have any questions please feel free to leave a comment.