This does depend a lot on your starting point, but I am going to give you general advice:
1. Get a good textbook; I had a great experience with Deutsch Na Klar! It also has a workbook with an answer key at the back, so you can really test yourself.
2. Set a pace at which you'd like to go through the book. I don't recommend spending less than three days on any chapter. Don't try to absorb everything in that chapter. Many times you will come across words irrelevant to the topic of the chapter; have a look at them, but don't try too hard to commit them to memory. It's important to keep focused. Go over that chapter a few times; look for materials on the internet relevant to that chapter, such as clips on Youtube about different jobs in Germany when you are learning about careers etc.
3. Watch German films, listen to German podcasts and German radio. Here's a website that actually gives decent suggestions and organizes all media by the level of German for which it is most appropriate: German Materials . Some of my favourite films for the level of German you are targeting were Good Bye Lenin, Viktor Vogel, and The Edukators. I also recommend searching for interviews with the actors in a film after you've finished watching the movie; interviews don't have complicated language for the most part, and they also repeat some of the main ideas of the film.
For radio, I recommend starting with Delta Radio Hamburg (delta radio) because they broadcast an approx. 2 minutes long news snippet every half an hour or so; usually, the snippet is the same throughout a whole day, so hearing it a few times really helps you comprehend it better.
Listen to German music. Some of my favourite German singers/bands are Die Toten Hosen (older rock band, highlights include Tage wie diese, Bonnie und Clyde, Paradies, Steh Auf), Peter Fox (sort of hip-hop, start with Schwarz und Blau, which is a great song about Berlin at night), The Wise Guys (a cappella, look up Jetzt ist Sommer, Nur für Dich), Die Ärzte (Ist das noch Punk-Rock), Die Prinzen (Deutschland). Again, listening to German songs has the advantage of exposing you to the same material over and over, thus really helping you retain more.
4. When you engage with all these German materials, don't be afraid to pause the film/podcast and to play it over again until you understand what they say. When you write down a word that you do not know, try to guess its meaning before looking it up in a dictionary.
5. Talk talk talk talk talk. Try to move to Germany or Austria for a short while, but avoid moving to an area where they traditionally have a strong accent; it's TOTALLY worth it, seeing how my German improved immensely in 6 weeks of living in Hamburg.
Otherwise, try to find some native speakers living in your area. or if there are none, there are some websites which offer conversation practice through Skype for a small charge. One cannot stress enough the value of practicing conversation when learning a new language.
6. If you want to read a book in German once you are around the A2 level, I suggest "Die verlorene Ehre der Katharina Blum" and the Harry Potter books in German.
Alternatively, you can go Thalia.de - Bücher portofrei bestellen (Thalia is a huge chain of bookstores in Germany), browse, and choose something that strikes your fancy; I don't recommend choosing anything above the Young Adult level, though, because of how complicated the vocab and syntax might be.
