Do you just use fluorescent pantone colours? And if so, how do you go about printing? Is RGB the correct file format? Even then, will I have to specify the colours used to the printing company? Also - where would you recommend I get the designs printed out? I want to make fluorescent posters and t-shirts, but I am a total noob at this whole printing thing.
Fluorescent Pantone colours are spot colours, i.e. special inks with fluorescent pigments.
Yes you would have to specify them. Your print company will need to make separations (separate screens) for each fluorescent colour in addition to any other colours. In Photoshop you would need to add a spot colour channel. Usually you would begin with a CMYK or Multichannel image, add a channel, specify the colour of that channel, and place the artwork in that channel. Read this for more info: https://helpx.adobe.com/photoshop/using/printing-spot-colors.html
Sorry, can't recommend "where" since you didn't mention "where" you are. But any screen printing company should be able to do it.
If it's for a poster, then lithographic printing would be suitable. It's basically the same in that separations for fluorescent colours would also be needed.