I don't claim to be an expert on this but I think I am qualified enough to give a few pointers. I can't reread more than a few chapters of the first fanfiction I wrote without cringing. Why is that? you might ask, well it's at least partially because at least one of the relationships in the story is extremely rushed. this is a common problem with stories here on wattpad, and if you want to avoid this consider the characters you are going to pair together, consider your character's personality traits,  what are your characters' like, are they shy or are they more of a social butterfly? factors like this should help you decide how far into the relationship your character should be before it progresses to the next stage. 

Another thing you should think about is your characters' beliefs or morals, I'm not specifically referring to religious beliefs, although they could also be a factor. you should decide if your character is open to having sex before marriage or not. Or on a similar note, would your character be willing to have kids before marriage or would they insist on waiting until after marriage? 

 Have they had previous relationships that failed? 

One problem with a lot of stories on wattpad is that the relationships are rushed when that would be out of character based on the character's personality traits and previous experiences. I know that everyone is different but I doubt a shy person would go from just meeting a person and then having sex not long after.  Someone who had a previous failed relationship not that long before probably wouldn't rush into a new relationship either. Sure sometimes people try to ignore their pain by being with someone else but consider your characters' personality. what would they do after a breakup?  

don't overdo it with putting together people who are opposites. sure these kinds of couples do work, but they are done so often that they are almost cliche.  couples who have more things in common can work just as well.  You can do whatever you want obviously it's your story these are just suggestions,  but I hope you take at least some of them into consideration.

While not everyone includes mature content in their books it's certainly pretty common. obviously, it's not something you have to do it's up to you. if it's not something you're comfortable with writing, you don't have to include it. Not everyone is comfortable reading it either.  While I have included mature scenes in my books before I put a warning before the scene starts so anyone who doesn't want to read it can skip it.  If you want to include a lot of mature scenes you should mark your story as mature since wattpad will probably do it if you don't. 

I'm not going to give advice on how to write sex scenes since that's something I'm still pretty new to myself. the last thing I'm going to say is your story doesn't have to be fifty shades of gray to be popular if you don't want to include sex scenes then don't.