Hello! First off, welcome to TamaTalk! Your English is just fine.
Great find! There's really nothing more to it than having the right screwdriver and maybe a little patience because it's like you said - it's mint on card and hasn't been touched in 15+ years.
Not sure about the exact screwdriver size/number but personally, I just bought one of the ones that come together with an eyeglasses maintenance kit. That should do just fine.
There are two screws that hold the battery cover closed in the back. When you're unscrewing the back, make sure to use firm pressure, as the screws could be a little tight. Also, vintage Tamagotchis are notorious for having soft screws and so they may become stripped if you're not careful.
As for the batteries, well, you always run the risk of leakage/corrosion if they haven't been replaced in years. Best to just remove them as soon as you get the unit open. If there is some leakage on the battery terminals, just use a Q-tip to remove it. If there's a bit of corrosion, baking soda paste works wonders. Just mix a bit of baking soda in water, dab a bit on the affected area (be very careful with this), wait a few minutes, then wipe off the residue with a damp cloth. Don't forget to let it dry completely before putting new batteries in.
Speaking of new batteries, your Tamagotchi will take two LR44 batteries. They're also called by different names depending on the manufacturer. Some of the common names are LR1154, AG13, A76, and 357.
Hope this helps!