Pesachim, Chapter 9, Mishnah 3. The Mishnah asks: what are the differences between the Pesach offering brought on the fourteenth of Nissan - the first Pesach - and the one brought on the fourteenth of Iyar, which is Pesach Sheni?
The differences between the first Pesach and Pesach Sheni:
The prohibition of chametz: On the first Pesach a person may not see chametz or keep it in his possession, and he may not own it at all. On Pesach Sheni, by contrast, matzah and chametz may both sit with him in his house, and there is no prohibition whatsoever against owning chametz.
Reciting Hallel while eating: The first Pesach offering requires Hallel at the time it is eaten, as we do at the Seder, whereas the Pesach Sheni offering does not require Hallel at the time it is eaten.
The laws that are the same for both:
Hallel at the time of the offering: Both require the recitation of Hallel at the time the Pesach offering is brought in the Beis HaMikdash, and on Pesach Sheni too it was recited there.
The manner of eating: Both are eaten roasted, together with matzah and marror.
Overriding Shabbos: Both override Shabbos, and they may be brought even on Shabbos.
The law of tumah: The first Pesach may be brought in a state of tumah when most of the community is tamei, but on Pesach Sheni, even if most of the community is tamei, it is not brought in a state of tumah. The reason: it was tumah that pushed the offering off to Pesach Sheni in the first place, and we do not say that it should now once again become permitted on the second occasion as well. Therefore, if the community is tamei, even the majority of it, the Pesach offering is simply not brought at that time.
Additional differences not listed in the Mishnah:
Three groups: The first Pesach offering is brought in three groups, whereas on Pesach Sheni this is not done.
The chagigah offering: Depending on the circumstances, a chagigah offering was brought together with the first Pesach offering, whereas with Pesach Sheni no chagigah offering is brought.
The Mishnah does not mention these differences, and as it does in many places, it lists some of the items and not all of them.
In summary: In this Mishnah we learned the differences between the first Pesach and Pesach Sheni - the prohibition of chametz and the recitation of Hallel while eating; the laws that are the same for both - Hallel at the time of the offering, eating it roasted with matzah and marror, and overriding Shabbos; the law of tumah, which does not apply to the second Pesach because it was tumah that pushed the offering off in the first place; and the additional differences that the Mishnah does not list - the three groups and the chagigah offering.