At the beginning? hehehe
No, seriously, that's an awesome point to raise. I've been writing original stories on a regular basis for years and beginning one still scares me. I have too much to say on this subject so sorry in advance for babbling.
You could start with a character, a situation or an event. It doesn't always work but generally when you have one element, the others are easier to find. Listening in to people's conversations (yeah, that's totally allowed if you're a writer) or reading the news can help. In fact, read everything, listen to everything, everything can be useful (I’m writing a story about a badger right now). Always be asking yourself 'what if...'. What if unicorns took over the planet? What if there was a world-wide food shortage? These tend to get either silly or really deep but if you ask it often enough, you'll probably find a concept that grabs you and possibly someone to inhabit it. If not and you’re incurably stumped, don’t be afraid of using online generators-I do it sometimes when I’m sick of the idea drought. You could also try roleplaying with yourself, putting yourself in several character’s heads in turn until you decide who feels most comfortable.
As for literally starting the story (and that can help when thinking about the rest), the bit of advice my tutors like to use is to tell you to think about how late you can start. So, if you're going to write about the titanic, can you start your story on the moment of impact with the iceberg or even in the terrified moments after it? If you're going to go the event route, don't be afraid of starting with that event. As for the character route, what point of their life can you reasonably start from? You could just have them waking up and getting coffee or you could begin with them fleeing from a crazed unicorn. Which is going to get you more excited? Remember, you can always go back and add detail or extra context in later.
Also, remember that this is only the first draft. You could just start by writing a few random, unconnected scenes from a character’s point of view. They might be useful in the overall story or they might not but at least you’ll get a feel for what you want to write. I tend to write chronologically (beginning to end) but I still have documents packed with unordered scenes I just had to write down for later.
TL;DR: Ask questions. Notice everything. Jump in to a story at whatever point you can, no matter if that's near the end or the beginning. Don't worry too much.
"Does the Walker choose the path or the path the walker?" -Garth Nix
"Without reading, we are all without light in the dark, without fire in the cold."-Tamora Pierce
(Av/sig by Amy!)