It is important that you research your topic well, people reading your books will often have a lot of knowledge on the subject area so it is important that you research your facts fully. It is probably a good idea to read a lot of science fiction books before you start so that you can get an idea as to how they should be written, and how others do this.
Most science fiction novels have a main conflict, as this is science fiction it can be a little different, make sure that you think outside the box. Deciding your setting and time frame is important, most science fiction novels are written in the near or far distant future and often involve more than just planet earth! You need to have a very good imagination and be creative and original with both your ideas and language used. The world you create for the reader needs to be convincing, it also needs to remain consistent throughout so that the reader finds it believable. It is becoming increasingly hard to invent new realities but with some care and imagination it is not impossible.
To help you with this, if you are not already a fan of the genre, make sure that you read past and present science fiction. As well as sparking off your ideas, it can help you to avoid inadvertently developing copycat realities. For example, a 1990s sci-fi series often seemed to have episodes that were rooted in existing films and books - after a while it lost its appeal as the ideas were not original.
A common mistake when writing science fiction is confusing it with fantasy. Fantasy novels are normally those that involve events that would be impossible to actually occur in real life, whereas science fiction is about scientific events that could technically happen within our world. Unless you are writing fantasy, make sure that any science is plausible although it has been said that science and magic are very close. Imagine someone from the dark ages seeing something as simple as an electric light being switched on... to us it is very understanding but to them it would seem like magic. But beware of making your novel just about the science and the fiction, make sure there is a solid plot in there too.
The characters, as in all novels are a vital ingredient. Make sure that they are believable and that the reader can relate to them. To achieve that, it is essential to plan and explore each of the key characters thoroughly. Work out their mannerisms, their way of speaking and how they communicate with other people very carefully.
So good luck writing your science fiction book, when the manuscript is finally prepared and you believe it is ready to submit it to the publisher, have it read and appraised by a professional service. The benefits are two-fold, firstly, it can help you to refine your novel and eliminate clumsy prose and secondly, having an error free manuscript with correct spelling, punctuation and grammar can help to keep your work off the slush pile. But beware of endlessly re-working your manuscript as there is the danger that it will never reach the publisher.