You're going to need Nifskope, Blender or 3ds max whichever you prefer, however Blender is free. Maybe outfit studio. You'll then need to fit that model on to either the fo4 skeleton or something. Tumbajumba on Nexus would be a good person to ask for that because he's made two custom power armors already. Also justice123 did the new NAVI power armor.
Here's the thing you need to know, this isn't going to be something you'll be able to slap together in a day, it's going to take you some time and hair pulling to get the hang of it. It's not going to be easy. My first attempt of trying to do a port was just that. I decided to shelf it because Fallout 4 was coming out soon. But be patient and stick with it and you'll make your idea a reality.
As far as the weapon, you should start with that, as I have seen people suggest starting with a weapon first to start learning things. You'll still need Blender or 3ds max for that.