I think the above replies explained well, and my additional comment might be useless, but let me try anyway.
I don't find very appropriate the terminology used in the learning of this usage. But, I will accept for the moment ordinary terms used most likely in Japanese language instruction (I myself have no experience of teaching Japanese to foreigners).
I know it is really hard to explain this entirely and clearly, because the use of the so called polite language (or honorific style, respect style, humble style, polite style, plain style) is immanent in the system of Japanese social and individual human relationships. Understanding it liguistically and using it adequately in your real life are different.
First, you, the OP, must make a clear distinction between the keigotai (honorific, respect and humble styles) and the futsūtai/teineigo (plain/polite styles). Because these are in different categories.
The keigotai is used premising the difference of social statuses (higher rank / lower rank), or the certain difference of age (older / youger), it is therefore used in fixed social relationships, it is not flexible, it is irreversible; you are therefore not allowed whether you use or don't use the keigotai style according to your feeling. It is beyond your free intention. It is required rigidly in your social life.
On the contrary, the pair of futsūtai/teineigo can be used on your intention, they are used among family members, friends, equals. With family members the plain style is usually used, and with friends and equals, both the plain style and the polite style can be generally used, and the transition from/to of these two style is, in a sense, flexible, especially among friends and equals who have a little difference of age. A regular transition from the polite (-desu/-masu) to the plain is common, but a reverse transition from the plain to the polite sometimes occurs, according to your mental state or your fantasy. It is therefore a psychological inward game between you and your interlocutors. Because it all depends on the feeling of mental closeness/distance, and this closeness/distance is not necessarily fixed, it is, I repeat, flexible and reversible.
So, as long as enough closeness is not established yet between you and your interlocutor(s), you must ordinarily use the polite form (-desu, -masu). This distance may turn, gradually or at one moment, to a certain mental closeness or familiarity, you can then use the plain style, and if, at your change of style, your interlocutor(s) feel as close as you do, the conversation will turn in natural way to the plain style. Or, you can make an implicit sign to your interlocutor(s) by using intentionally the plain style at a moment. Anyway, unless both sides feel the same level of closeness, your relationship with your interlocutors is still unsteady, and it would be rude for you to use the plain style in one way. It is very possible that your interlocutors don't feel as much closeness as you, because you are for the present nothing but temporary acquaintances, and that they are not mentally ready for your familiarity. In other words, it might be also possible that, by continuing to use the polite style, they blame you inward to be rude or ignorant.
I also wondered how old you were, because the discussion I made above is not necessarily true among young school students and even college students, who are considered as socially immature, but it is true among adults, who are no more under educational protection. So...
As for using "ore", I have my opionion, but it's another story.
Be persevere in your Japanese language learning.