This often involves a character staring at themselves in the mirror and wondering how, exactly, they ended up there. They think about their situation, what has happened, how they feel about it all. And all the while the reader is thinking: where are they? what's happening? why is this interesting? We know characters by their thought processes, but we need to establish some basics first such as what's going on (ie action), where are they (physically and temporally), what the story problems are before we go into the workings of their mind too deeply.
2. Gimmicky Action
The opposite of the internal monologue, this begins with nothing but action and of the most dramatic sort - car crashes and chases, exploding this and that. The trouble is, until we know the characters we don't care what's happening to them. 'Normal world' has to be established before we can leap into action. Closely related to Gimmicky Action is...
3. The Info Dump: Factual
This is when the writer thinks we need to know lots of facts about the characters and their situation before we can understand them. Not true: we can know very few facts about characters and their situation, so long as we understand the emotional meaning or resonance that they have. But that's not to say a better idea is...
4. The Info Dump: Emotional
A few weeks ago I was sitting in a cafe having cup of tea when I fell into conversation with another woman. She then told me her life story including a lot of traumatic detail about her miscarriage and how she'd decided not to have another child. The writer part of me was fascinated, and as a woman I was sympathetic, but it wasn't a comfortable situation. Piling on loads of heavy emotional stuff right at the beginning will have the same effect on the reader. You may think it's a way of getting sympathy for a character, but most readers will find it cringe-making. Again, establish 'normal world' first.
You may have noticed I'm not a fan of flashback generally but I particularly don't like it on the first page of a novel. It makes 'normal world' hard to place - is normal the present situation, or the one in the past? You want the reader to be swept up into the story of what it happening now, and flashback, by definition, is not now. Plus, it can be confusing, and the last thing you want to do is confuse the reader right from the start.
So what do I think are good ways of starting a novel?
1. Establish normal world
2. Within normal world hint that all is not as it seems.
3. Show characters in action
4. Give an indication of the main theme(s) of the novel
5. Clearly show the genre (romance, thriller, literary, sci-fi etc)