Recommend level 80 character for boost (PVE)
You can use your L80 Boost on a trial basis in The Silverwastes for each and every Profession before deciding which you prefer.
Here’s a video with instructions: https://www.youtube.com/watch?v=7IbMbGG1lOU
Good luck.
There is no right or wrong answer with it other than to not use it when first starting out on GW2 for the first time. I ended up using mine on mesmer to avoid the squishy leveling process and jump right to grouped PVE/WVW where mesmer does best. Maybe try it out on a Medium or Heavy class? Either way you will need to rack up some Hero Points to unlock the elite spec. on your new character. If you are interested in running the HoTs maps tonight PM me as my guild is hosting an event for HoTs HP and such.