I'd say start with the classics of the alien novel genre. Then move on to the newer releases. That way, you get a sense of the roots and the contemporary takes. But really, it's up to what catches your eye and makes you want to dive in.
Well, you might consider reading them in chronological order of publication. This way, you can see how the themes and writing styles evolve over time. But it's not a strict rule. You can also choose based on the author's reputation or the summary that interests you the most.
It depends on your preferences. You could start with the most popular or highly-rated ones first. That often gives a good introduction to the genre.