One possible scenario could be Sirius noticing how Harry blushes whenever Ginny's name is mentioned. So he might say something like, 'Harry, I've seen you go redder than a Weasley's hair at the mention of Ginny.' Another could be if Ginny gives Harry a special gift, and Sirius teases him about it, 'Harry, that little present from Ginny must mean something special, eh?' A third scenario could be when Ginny is around and Harry acts a bit clumsy. Sirius would then joke, 'Harry, you're more out of sorts than a drunk Hippogriff when Ginny's near.'
Well, perhaps in a fanfic, Sirius sees Harry writing a letter to Ginny. He could then tease him by saying, 'Harry, writing a love letter to Ginny, are we? I bet it's full of all sorts of sappy things.' Or if Ginny is on the Hogwarts Quidditch team and Harry is watching her play. Sirius might say, 'Harry, you're watching Ginny fly around like she's the only one on the pitch. It's quite obvious you've got a thing for her.' Also, if Ginny is laughing at one of Harry's jokes in a group setting, Sirius could say, 'Harry, Ginny seems to find you awfully funny. Are you trying to charm her with your humor?'
In some fanfics, Sirius might tease Harry about Ginny when they are all at the Burrow. For example, if Ginny is helping Molly in the kitchen and Harry keeps stealing glances at her. Sirius could nudge Harry and say, 'Stop ogling Ginny, Harry. You're not being very subtle.' Or if Ginny is showing off a new spell she learned, and Harry is clearly impressed. Sirius might say, 'Harry, you look like you've just seen the most amazing thing in the world, and it's just Ginny doing a simple spell.' Another instance could be when they are all sitting around the table, and Ginny and Harry accidentally touch hands. Sirius would chuckle and say, 'Well, that was quite the touch, Harry. What's going on in that head of yours regarding Ginny?'
Harry might blush and stammer. He could say something like, 'Sirius, it's not like that.' and then quickly try to change the subject.
One possible scenario could be that Harry finally finds the courage during a quiet moment at Hogwarts, perhaps in the Gryffindor common room by the fireplace. He might stutter a bit at first, but then clearly say his feelings. Another could be that it happens during a dangerous adventure when he realizes he can't hold his feelings in any longer, fearing he might lose Ginny. Maybe he confesses after Ginny has been in some sort of danger and he realizes how much she means to him.
One possible scenario could be that they are in the Gryffindor common room. The fire is crackling, and everyone else has gone to bed. Harry and Ginny sit on the couch, and slowly start to get closer. Their eyes meet, and then they share a gentle kiss that gradually becomes more passionate as they let their feelings for each other take over.
One popular 'sirius teases harry fanfic' might be 'Pranks and Protections'. In this fic, Sirius uses his mischievous nature to tease Harry in a light - hearted way. For example, he might transform into different animals to startle Harry and then laugh it off. Another could be 'Family Bonds', where Sirius teases Harry about his crush at Hogwarts while also giving him valuable advice on how to handle relationships.
A great scenario in a Ginny and Harry get married fanfic would be a destination wedding in a far - flung magical place. Maybe they choose a wizarding island where they can have the ceremony on the beach under a starlit sky filled with magical constellations. Another scenario could be that they get married in a rebuilt Hogwarts, with the castle being a symbol of their past and their future together. And then there could be a scenario where Ginny and Harry have a very modern - wizarding wedding, with new spells and charms being used for the decorations and entertainment that no one has seen before.
One possible scenario could be that Ginny and Harry are in the Gryffindor common room. Ginny has had a long day of Quidditch practice and studying. She sits next to Harry on the couch, and as they are chatting softly about their day, her eyes start to droop. Harry notices but doesn't want to disturb her, and soon she falls asleep on his shoulder. Harry then just sits there, feeling a warm sense of protectiveness and contentment.
You can try fanfiction.net. It has a large collection of Harry Potter fanfics, and there's a good chance you'll find some 'sirius teases harry fanfic' there. Just use the search bar and type in the relevant keywords.
Perhaps in the fanfiction, Sirius gets involved with the wrong crowd. And Harry, who has a strong sense of right and wrong, can't stand being associated with Sirius any longer. Or maybe Sirius has a falling out with one of Harry's other important people, like Hermione or Ron, and Harry takes their side and starts to hate Sirius as a result.
One possible scenario could be that Ginny meets an old acquaintance who has changed a great deal and she starts to have feelings for this person while still being with Harry. Maybe they share a common interest that she feels Harry doesn't understand. However, this goes against Ginny's strong character in the original series, so it's a very different take in fanfiction.
In some versions, it could be that Harry has to die to break a dark curse placed on Ginny. He discovers that his life is the key to saving her and willingly gives it up. He might have a premonition about Ginny's fate and decides that his death is the only way to change it. He then confronts the source of the curse and allows it to take his life instead of Ginny's.