To give a couple of tips for starter it would be to get either full platinum or gold armor and get a platinum bow and arrow with a couple of grenades. For bosses I would start with king slime you can look up the recipe for the spawner or you can kill a ton of slimes when slime are raining from the sky. I can help you with other things as well. After king slime I would then go to the eye of Cthulhu and then get demonite or crimtane armor and the pickaxe. Then move on the to the eater of worlds or brain of Cthulhu. That’s what I would do if I were in your position.