There are two types of Sichos:
Sichos Kodesh / Toras Menachem-Hisvaaduyos.
From the early forties until the stroke in 1992, the Lubavitcher Rebbe would hold a Farbrengen (at least) every Shabbos Mevorchim and other special days, which (after the passing of his predecessor R' Yosef Yitzchak Shneersohn in 1950), were transcribed and printed. Originally transcribed in the original (Yiddish), in 1982 the committee in charge publishing those talks translated into Hebrew as Hisvaaduyos .
Since there's a preference to learn those Sichos the Lubavitcher Rebbe prepared for publication, and since not every Parsha has a Sicha every year (for example, there is no Sicha on Noach in 1951), people tend to learn Likkutei Sichos more often than Sichos Kodesh.
Likkutei Sichos (the Sichos which were edited by the Lubavitcher Rebbe for print).
Starting in the 70s or so, the Lubavitcher Rebbe edited a Sicha almost every week, publishing them as Likkutei Sichos pamphlets, then every few years they would be gathered together into books, arranged by Parsha (a chumash per volume).
Generally, Likkutei Sichos tend to be broken down into the following categories:
- "Farbrengen style" - most similar to a Drasha, take a verse, expound it in a Chassidic way, and show how it applies to day to day life. These tend to mostly found in the first four volumes.
- Chassidus - The Lubavitcher Rebbe says pilpulim in Chassidus.
- Rashi - The Lubavitcher Rebbe says pilpulim on Rashi
- Rambam/Hadrans - The Lubavitcher Rebbe says pilpulim on Rambam or on a Masechta.
Generally, the first four volumes tend to flow more like a Farbrengen, while the rest tend to have a mix (though the 30-39 tend to be more focused on Niglah or Rashi).
So in terms of content, the first 4 volumes tend to be the easiest, especially for a child or beginner.
In terms of language, all of Likkutei Sichos are in Yiddish, except for volumes 10-14 (one full cycle) and 30-39 (one and a half full cycles), most of those tend to be more in the style of Pilpul. However, there are tools to learn Sichos written in Yiddish for those who don't speak the language:
- There is a set of Sichos translated into Hebrew,
- The Likkutei Sichos dictionary (which contains all Yiddish words found in Likkutei Sichos [what's nice about it is that it lists words with prefixes and suffixes, so you don't have to guess roots])
- "Back to Basics" - a book which translates some Yiddish Sichos (mostly from volume 1 and 2), and has a small dictionary itself.
. Until 1988, when the Lubavitcher Rebbe stopped holding Farbrengens during the week.
. Around 1992, Lahak/Kehot started the huge work of re-editing and translating them into Hebrew, also publishing it as Toras Menachem - they are currently up to 1972, so they have about 10 years of Sichos left).
. Sichos in English is a partial translation of these Sichos from 1977.
. The first four volumes where printed earlier and had a different format: Volume one and three contains Bereishis, Shemos and Vayikra and Volume two and four contains Bamidbar and Devarim.