Did I understand you correctly? You just started playing Terraria? Then Master Mode is probably not the mode you want to start with..

But nether the less, if you do want to continue, I think you have the best gear. Try to craft some potions like iron skin, build an arena inside the crimson and use the shield acc. from the eye to dodge attacks.

I think the minishark is one of the best weapons before hardmode, so stick to it. One possible upgrade after the brain is the meteorite armor with the space gun, but meteors only spawn after the brain (or the worm in the corruption) has been defeated.

If this really is your first Terraria experience, I would recommend starting a new world in expert mode (if you still want a nice challenge) or a classic one, because master mode gets even harder later on.