Acquire an audio reference to ensure you are pronouncing the numbers properly. Look into audio books, DVDs or learning software. Rosetta Stone is a popular learning software, but also expensive (see Resources).
Practice numbers as you learn them and then on a regular basis to ensure maximum retention. Mix up the learning a bit to make it more fun. Try translating phone numbers or solving basic math problems using only Portuguese numbers. Make a deck of cards with Portuguese numbers and play card games with them.
Learn the cardinal numbers (1, 2, 3) before attempting ordinal numbers (first, second, third). Once you have the cardinal numbers memorized, ordinal numbers will be easier to learn.
Make flash cards to test yourself and break up the learning experience.