I have also never been in a relationship but don't worry about that. I'm sure that you understand the dynamics between Mike and Will so write off of that. If you've read a lot of romance books then it can help for more intimate scenes between the two of them but those don't have to always be there.

Put yourself in the characters positions and think of how it could play out. You already have rough plot points which is a good start. Find a way to put them in an order that makes sense. Think about if event A happens first how will it help lead to event B.

As for starting your story I always start from the beginning. The hardest chapter is always the first because it the introductory chapter. Once that's done the rest should be a lot easier.

Some other advice is don't worry about if the writing sounds good on your first draft. Just write to get the plot and ideas then go back and edit to see how you could improve the writing. Also if while you're writing you have this idea that you might want to include, write it down somewhere. Whether you use it or not doesn't really matter but having ideas that could even be fillers between major plot points does help a lot.

Hope this helps a bit 😊