How are the importance of sutras determined?

Every one of the 18,000 plus suttas are important in their own way as the Buddha taught the Noble Eightfold Path to his followers according to a "gradual" system of training. We are all at different levels and none are at the same level in this road less travelled. The Paccattam veditabbo viññuhiti quality of Dhamma is to be realized by the wise, each for himself. So one sutta will speak for one, but not the other. But there are two Suttas in the Anguttara Nikaya that will apply to all. It is the Pathama Lokadhamma Sutta and the Dutiya Loka,dhamma Sutta. They are about the Eight Worldly Conditions (Vicissitudes) Of Life. Everyone of us sorrow when we face of our everyday sufferings and troubles, and we joy when we face the moments of pleasure and happiness. There are eight such vicissitudes of life (ata lō dahama). We are slaves of the vicissitudes of life. These sermons help develop a better range of strengths to deal with these eight vicissitudes of life. So please read:

http://www.themindingcentre.org/dharmafarer/wp-content/uploads/2013/04/42.2-Lokadhamma-S-1-a8.5-piya.pdf http://www.themindingcentre.org/dharmafarer/wp-content/uploads/2013/04/42.3-Lokadhamma-S-2-a8.6-piya.pdf

If you seek Advice on the Practice, suttas such as Bhaddaali Sutta MN 65, Bodhiraja Kumara Sutta MN 85, Chathuma Sutta MN, Chula Sakuludai Sutta MN, Dhamma Dayada Sutta MN 03, Ganaka moggallana Sutta MN 107, Giminananda Sutta AN 10.60, Hatthigamaka Ugga Sutta AN 08, Lomasangiya Bhaddekaraththa Sutta MN 131, Maha Rahulovada Sutta MN 62, Sakkara Garukara Sutta, Samadi Bhavana Sutta AN 4.41, Sambodhi Pakkhiya Sutta, Vasijatopama sutta would be helpful.

If you want to learn about Skillful & Unskillful Thoughts read Kalama Sutta AN 3.65, Kodhana Sutta AN 7.60, Kukkuravatiya Sutta MN 57, Maha Dhamma Samadana Sutta MN 46, Maha Vacchagotta Sutta SN 44.8, Malunkaya Putta Sutta SN 35.95, Sakka Panna sutta SN 35.118, Vassakara Sutta AN 4.35, Vitakka Santhana Sutta MN 20.

If you want to read on Skillful, Unskillful Actions the following suttas will do: Chakkavatthi Sihanada Sutta DN 26, Gopalaka sutta AN 11.18, Ina Sutta AN 6.45, Jivaka Sutta AN 8.26, Kandaraka Sutta MN 51, Lakkana Sutta AN 3.2, Subha Sutta MN 99.

Then there are Suttas aimed at Life of a Layman. Chalabijathi Sutta, Devadaha Sutta MN 101, Gayha Sutta SN, Janussoni Sutta AN 10.177, Jara Sutta SN 48.41, Mahanama Sutta AN 11.13, Patama Nathakarana Sutta AN, Pattakamma Sutta AN 4.61, Sallekha Sutta MN 8, Sattabharya Sutta, Sigalovada sutta DN 31, Sihasenapati sutta AN 5, Vyaggapajja Sutta AN 8.54.

On Intentional action_Kamma you can check out the suttas: Cula Kamma Vibanga Sutta MN 135, Hemavatha Sutta KN, Lonaphala sutta AN 3.99, Maha KammaVibanga Sutta, Mallika Sutta SN 3.8, Purisa Indriya gnana Sutta.

Then on the seven_enlightenment_factors - Kundaliya Sutta SN 46.6
And on the Noble Eight Fold Path (Arya Ashtangika Margaya) - Maha Chattarisaka Sutta, Sachcha Vibhanga Sutta MN 141, Samma Ditthi Sutta MN 9,

In Pali Buddhism, the original sole purpose of the teachings was to end suffering. However, overtime, certain suttas were introduced to promote morality in laypeople (which emphasise kamma & 'rebirth') & to create propaganda against & convert people from Brahmanism.

Therefore, in Pali Buddhism, the most important suttas are those connected with Emptiness (sunnata) & the ending of suffering, such as the First Three Sermons of the Buddha on the Four Noble Truths, the Three Characteristics & on the Fires of greed, hatred & delusion.

Ultimately, the only suttas that can be deemed to be fully authentic are those that accord with the principle of 'paccattam veditabbo viññuhiti', namely, to be realized by insight, each for himself.

Thus you should train yourselves: 'We will listen when discourses that are words of the Tathagata — deep, deep in their meaning, transcendent, connected with emptiness — are being recited. We will lend ear, will set our hearts on knowing them, will regard these teachings as worth grasping & mastering.' That's how you should train yourselves.

Ani Sutta

~~

What I have revealed is: 'This is Suffering, this is the Arising of Suffering, this is the Cessation of Suffering, and this is the Path that leads to the Cessation of Suffering.' And why, monks, have I revealed it?

Because this is related to the goal, fundamental to the holy life, conduces to disenchantment, dispassion, cessation, tranquillity, higher knowledge, enlightenment and Nibbaana, therefore I have revealed it.

Simsapa Sutta: The Simsapa Leaves

There are accounts or records of Buddhist councils which were convened to determine (and record) what suttas were true/accurate/well remembered versions, of discourses which were spoken, heard and remembered during the Buddha's lifetime, and which were then passed on by word of mouth.

I think that the Theravada school maybe doesn't see the Lotus sutra (nor the Flower Garland nor any 'Mahayana' sutras) as "canonical" -- I don't know why that is: my uninformed guess is that it's because the authorship/provenance of those sutras is too late (and maybe too distant geographically), and the intent of the Theravada school is to preserve the 'original' teachings as far as possible; and maybe that there are 'enough' Pali suttas, and that the original teachings are seen as sufficient (because for example there's a record of teaching being effective during the Buddha's lifetime).

See also Pāli Canon and Āgama and Sutta Pitaka.

Think about martial arts. One'd be a lot more reserved before walking into any dojo and claim that his style is the one true style. Why? because he knows that his claim will sure be put to the test inside the ring. And until he walks out of that ring alive and on two feet then all his words don't mean a thing. Just like that, the Buddha Dhamma is there for a purpose, not for mere talk. He actually emphasized that when instructing Mahāpajāpatī Gotamī in AN 8.53. Anyway, as you mentioned you have little knowledge with Buddhism outside your own sect and you're interested in how other parts of the Teaching works, I'd strongly recommend you checking out accesstoinsight.org and suttacentral.net. The "Befriending the Suttas" link is particularly useful for the Buddha's original teaching is there and treasured by all schools (the Theravada's Nikayas and the Mahayana/Tibetan's Agamas):

"Gotami, the qualities of which you may know, 'These qualities lead to passion, not to dispassion; to being fettered, not to being unfettered; to accumulating, not to shedding; to self-aggrandizement, not to modesty; to discontent, not to contentment; to entanglement, not to seclusion; to laziness, not to aroused persistence; to being burdensome, not to being unburdensome': You may categorically hold, 'This is not the Dhamma, this is not the Vinaya, this is not the Teacher's instruction.'

"As for the qualities of which you may know, 'These qualities lead to dispassion, not to passion; to being unfettered, not to being fettered; to shedding, not to accumulating; to modesty, not to self-aggrandizement; to contentment, not to discontent; to seclusion, not to entanglement; to aroused persistence, not to laziness; to being unburdensome, not to being burdensome': You may categorically hold, 'This is the Dhamma, this is the Vinaya, this is the Teacher's instruction.'"