This is just something you have to learn. Usually, before a vowel, it's "an" in English, but with a vowel that sounds like a consonant it's "a". "U" in "useful" is pronounced like the word "you". Not "oo", as in "moon". So it's a vowel sounding like a consonant - use "a".
The translation is easy, but I find myself doubting my translation of odd sentences like this, because nobody would ever say this. This is giving a specific description of a general thing. More likely they would say "Ik heb een handig apparaat" or "Ik heb een handige kaart".
Because useful is pronounced as if it starts with a consonant, hence it's a useful.