1) Get a round cut. They are always in style. Simplicity is better, especially if the diamond is nice.
2) Get the biggest one you can while only being able to see whatever flaws it might have through a loop.
3) Whatever you think you can afford, get an even better diamond. My bro got married young, bought what he could afford at the time. Then he started crushing it and always wished he had gotten something bigger/better.
He could do that now, but most women worth their salt are emotionally tied to that first diamond from what I've seen.
Ask some trusted friends. Most guys who have been engaged probably know someone. I'd stay away from a Jared's, unless I knew the owner.
If you want to DM me, I have a great guy. He helps me whenever I need pieces. He's in Oregon of all places. Typically doesn't do one off stuff as he moves weight, but he does it for friends.
Talk about a crazy world. Few like the diamond biz.