Well, I have RFID experience based on my very complex RFID project which I published on Codeproject:
I recommend you to read my article. It uses a PN532 chip from NXP to communicate encrypted with a Desfire EV1 RFID card. My project is a high security door opener which stores encrypted data on the memory of the RFID card.
From my experience with RFID I can tell you that with normal hardware you will never get 20 - 30 cm.
The easiest way for you would be to re-think why you need so much distance?
If you can get the smartphone nearer (less than 5 cm) you can use standard hardware from Adafruit with the PN532 chip which also does NFC communication (both: active and passive). My project does not use NFC, but it is similar.
But for more than 5 cm you will need a bigger antenna and a HF amplifier. This will be much more work. So I really recommend you to re-think why you need so much distance?
From NXP (former Phillips and the vendor of the PN532 chip) there is an annotation "RF Amplifier for NXP Contactless NFC Reader ICs" AN1425.pdf which describes how to amplify the HF signal.
I attach an image from this document.
But I can tell you that building a HF amplifier is nothing for beginners. You have to calculate the antenna, coils and capacitors and design a PCB. If you don't have experience with HF electronics, simply forget it.
Additionally to the hardware you will have to write the software, which also is not easy. RFID is a very complex thing! I spent several month in making my project.