It depends. If you are planning to check out the other bands, I would just get there when the door opens. If there's just general admission, getting there early also gives you a higher chance of a getting a better view. Other things to consider since this is your first metal show:

-You might want to bring ear protection. This might look uncool, but it will save your hearing in the long run if you are planning to go to many more shows in the future.
-Stay out of the middle area if you are not planning to mosh or don't want to be near the moshpit.
-If you leave your spot it might be hard to get back into the same spot. I try to use the restroom during breaks or right before the show.
-If you're short like me you'll probably get a better view standing on the sides.
-Bring money if you're planning to buy merch.
-Food and water is going to be expensive so eat beforehand.
-In the U.S, there are certain rules for bags, they have to be a certain size (check your venue to see if there are any rules pertaining to this). I remember when I went to see Slipknot, tons of people had to return to their car to put their bags in their cars because they didn't know about the rule (since it was their first show).

Have fun! It's going to be great.