Certain words are either masculine or feminine, and certain words are neutral - "de" is associated with the masculine and feminine words, and "het" with neutral words. Each word has its grammatical gender and so is associated with a certain article (though everything uses "de" in plural). Hope I could help!
'Lekker' is mostly used for something delicious. Like: 'it tasts delicious' would be 'het smaakt lekker'. What you imply, or rather what your informants has said it would only be used for someone who is good looking is totally wrong. It is rude to say to someone who is good looking that he/she is 'lekker'! But there are some who use 'lekker' for that, but it is not accepteble for everyone!