How can you learn the notes?

Learning first what single notes smell like and then after a while of smelling frags you will be able to pick up on different notes. There is a forum on single notes below this forum on single notes. I forget it's name, but you can read the descriptions and find it. It is very helpful!

I think you have to have a special knack for it, which I don't have.

While you can try to deduce notes from reading about the individual notes, reading reviews and then searching for the notes in the fragrance, or finding two fragrances that both share a note and trying to see what they have in common, this is a haphazard approach at best. Especially with regards to reviews - it's often a case of the blind leading the blind. Most people simply don't know what guaiac wood smells like, for instance. I don't blame them - it's still VERY difficult, and sometimes impossible - to tease apart the components of a scent even when you do know what the various ingredients smell like. Besides, there are molecules out there that you simply won't have access to unless you are a perfumer for a big company - so you'll never learn what they smell like in isolation.

I've got experience with a few synthetic aroma chemicals and about 200 natural materials. Really, the only way to really learn the notes is to experience them in isolation. I'd suggest a beginner's kit from Perfumer's Apprentice if you're serious about it. Otherwise - or rather, regardless - you can learn certain notes such as many spices from the spices you can buy at the store. (Note, however, that the synthetics, and natural essential oils and absolutes and C02 extracts often smell different than the fresh/dried spice.)

Flowers, too, you can sniff fairly easily. Head to a local gardening store/greenhouse and sniff away!

It is a matter of experience and practiced observation. It does help to start off with some notes that are very familiar, such as vanilla or clove or coffee. Sometimes certain funky shops will have scented oils and then you can get a direct sniff on specific note, although in practice it can be difficult to pick out notes in a complex fragrance.

It is necessary for a trainee perfumer to develop his sense of smell in two ways; one, to develop an odour memory ( so that he can remember what substance X smells like without going to the bottle and actually smelling it) and two, to develop an analytical sense of smell(so that he can detect individual notes in a mixture). Neither is easy to develop; it tales time and constant practice. The only way to develop the analytical sense of smell is to smell many individual materials, and learn them. Perfumers are constantly smelling stuff to remind themselves and maintain the memory. Start with synthetics, they tend to be less complex. Dip each one, smell it and describe what it smells like to you. It doesn't matter how daft the description is if it is relevant to you. Try to follow the evaporation of the material to work out if it is a top note, or middle note etc. Make notes. And keep on doing it until you can recognise the material blind. If you have never smelled something before you cannot detect it is a mixture. If you want to be able to do this, you will have to learn your smells.

Also, smell everything: flowers and herbs, spices on your spice rack, the beer and coffee you drink, etc. Many health food stores sell essential oils and there is often a tester vial open for you to try. Next, as everyone says, smell lots of perfumes. Take notes. Smell a number of things that have the same dominent note and notice the similarities and differences.
Above all, try perfumes repeatedly so that they have time to find an angle of repose in your memory. Not absorbing new smells at once doesn't mean there's anything wrong with your nose or your memory. It takes time and repetition.

Either knowing the notes from real life (roses, jasmine, cinnamon, clove and so on) or by smelling essential oils.

Smelling essential oils helped me recognize a bunch of resins I could not smell in real life.

It is only experience: when you know what are the notes in a fragrance you must learn to detect the single notes. After several times it will be easier and you'll master it.

Pick some perfumes that have a very strong particular note. for example Ambre sultan for ambre. Montale White oud for Saffron, etc. I have the BN top/mid/bottom notes pyramid printed out and hence I refer to it each time i spray and go through my wardrobe.

I've been trying to get a hang of it too. basically you need to expose yourself to the different notes before you will be able to identify them. reading reviews and searching for the known notes within a particular fragrance can also help train the nose quite abit.

but don't overdo it though, too many scents all at once can confuse you and make it hard to differentiate the new scent from the previous one. i find that a whiff of coffee in between smelling different fragrances helps you to drown out the earlier notes and get a "fresher" perspective on the scent profile. at least that works for me.

Just like with learning anything else; a little bit of talent and a whole lot of practice.

Sorry, where did you buy your essential oils?