You should start with the inner structure (the black parts on the first page of the instructions, also see build photos on the model page, in this thread or in this one:
X-wing in 1/48, another one)
The missing tabs - I used edge gluing for my build (you apply the glue just to the edge of the paper - it's stronger than you may think), or you can cut a paper strip from a separate sheet of paper and glue that from backside (where needed).
This may sound a bit rude, but if you are a beginner, it would be perhaps better to find an easier model to practice on first.