'Denne' and 'dette' both mean 'this', however denne is both masculine and feminine, and dette is neuter. Feminine nouns can be treated as masculine.
denne bilen - this car (masculine)
denne jakka - this jacket (feminine)
dette huset - this house (neuter)
disse bilene/jakkene/husene - these cars/jackets/houses (plural)
Remember also that when you use denne/dette, the noun must be in definite form.
en bil ... bilen - a car ... the car
en/ei jakke ... jakken/jakka - a jacket ... the jacket
et hus ... huset - a house ... the house
Hope this helped :)