There are some patterns to keep in mind which will help you remember the numbers 11-19 in Spanish.
-You do need to just memorize the numbers 11-15, but it may help to remember that they all end in –ce. They are: once, doce, trece, catorce, quince.
-The numbers 16-19 are very easy because they are all basically like “ten and six,” “ten and seven,” etc. Be careful of some small spelling changes:
diez (10) y (and) seis (6) = dieciséis (16)
diez (10) y (and) siete (7) = diecisiete (17)
diez (10) y (and) ocho (8) = dieciocho (18)
diez (10) y (and) nueve (9) = diecinueve (19)