OP: I played an gnome arcane trickster in EA and it was one of my favorite characters. I wouldn't worry too much about planning beforehand. You seem to have a basic knowledge of DnD 5e, that should be enough.
This. You apparently have D&D knowledge/experience and the game is made to be easily playable by people who have
none. You're even easily able to get your Arcane Trickster by just playing Rogue and picking Arcane Trickster as your subclass on level 3 iirc. Though you'll want some Intelligence to go with your Dexterity then obviously. Class and race will indeed make differences in dialogue but mostly in a positive way as far as I've seen, i.e. giving additional options or flavour. So don't worry too much. It certainly doesn't hurt thinking ahead but it's definitely not necessary.
