Hey Serena! It's definitely a good idea to have your whole story written before you begin posting, that way you don't have to worry about not knowing how to end it. Also, as you write you'll probably end up writing things that you'll throw out and have to start over cos they lead you down dead ends. This happened to me a LOT before I decided to put Stolen Hearts online. You don't want that stuff already posted when you realize it's not going to work! LOL!
I'm not sure what part of dialogue is giving you a hard time, but one of the things that helped me a lot was reading books and seeing how the authors wrote dialogue. Also, try not to have your characters say something they've already thought. That's a classic mistake I see here all the time.
If present tense is giving you a hard time, I would suggest third person which is how I wrote Stolen Hearts. I think it's easier to write in some ways. It also allows you to shift to different POV's if that's part of your story.
I think your story idea sounds good. There's a lot of emotions in a story like that and also tension, wondering what decisions are going to be made and if things will work out. If you want to make sure readers don't lose interest and can't wait to read the next chapter, keep them guessing what's going to happen next. I've read a lot of books on here I was excited about and then been disappointed because I knew within the first few chapters exactly what was going to happen.
Finally, and most importantly, when you're writing, make sure you write for you and not for anyone else. If you love what you writing, someone else will too! It makes writing much more enjoyable, which is the whole point! If you have any other questions, feel free to PM me! Have a great Valentine's Day!
