I know little to nothing about Korea except for the war and now it is divided. I would love to learn more about...well, everything really. Was there always a north/south tension/rivalry? What was the capital? What are some important historical milestones and landmarks of the country? What caused the rift? That's just to name a few questions...thanks in advance.
Get a book and read. You're basically asking for the history of a people that dates back 5,000 years (a little over 2,000 if you want accurate, verifiable history).
I would recommend Kyung Moon Hwang's A History of Korea. It's a quick read, each chapter is no more than 10-15 pages, and it's entertaining. It's a very broad history, but not very deep. Each chapter focus' on a specific, notable point and usually has some stories about notable people, so you'll come away with a lot of neat trivia. If you want more in depth details, Michael Seth's A History of Korea (very original titles) is a solid college level text on Korean history.
As to your specific inquiries:
Was there always a north/south tension/rivalry? No, not really. There was always a Three Kingdoms rivalry. Korea was originally dominated by three (actually more, but the minor ones were very minor) kingdoms, the Silla (southeast), Baekje (southwest), and Goguryeo (north). They were all unified under Silla in the mid 7th century which then ruled most of the peninsula (Balhae ruled the far north) until the Later Three Kingdoms period and then the Goryeo period, which lasted until 1392. In 1392, the Joseon dynasty began and ruled until Japan colonized Korea in 1910. Korean rivalries have always been regional. The southwest of the country, the Honam region (present day Jeolla provinces, historically the southern Baekje area) was neglected by the central government for most of the 20th century, which was ruled by people from the Gyeongsan region, the southeast (present day Gyeongsan provinces, Silla area). So there's always been that, but not a North Communist/South Capitalist sort of thing.
What was the capital? Seoul. The Silla capital was at Gyeongju, the Baekje capital was around Seoul, as well as Gongju and Buyeo in present day Chungnam province, and the Goguryeo capital was in Jilian province China, and later, Pyongyang. The short lived Taebong dynasty (901-918) had the capital at Kaesong and across the Han river at Ganghwa Island, the Goryeo dynasty holed up during the Mongol invasion of the 13th century.
What are some important historical milestones... the Joseon dynasty was founded in 1392. It ruled until 1910. During that time, King Sejong introduced hangul, the Korean alphabet, the Japanese invaded during the Imjin Wars of 1592-1598, and Korea had a policy of isolationism during the 19th century. The Japanese invaded Korea in the 1890s as part of the Sino-Japanese War, then again in the early 20th century as part of the Russo-Japanese War, and eventually colonized Korea. Korea was liberated in 1945 when Japan lost World War 2.
...and landmarks of the country? It's very mountainous. Baekdusan in North Korea and Hallasan on Jeju Island in South Korea are the tallest mountains in Korea and are both volcanoes. There are lots of old fortresses (Hwaseong Fortress in Suwon), Buddhist temples (Wolgyesa), palaces (Deokso, Changdeok, Gyeongbok), and natural areas to see.
What caused the rift? The cold war. The soviets occupied the north after WWII and the Americans occupied the south. The UN organized elections in 1948 for a unified government, but the north boycotted them and competing governments were established. In 1950, the north attacked the south in an attempt to unify the peninsula.
I'd start with Korea: Old and New by Carter Eckert. It'll give you the basics on Korean history since the stone age.
Was there always a north/south tension/rivalry?
Yes, there always seems to be some acknowledgement that the north is different from the south, but where that line is drawn has traditionally either not existed, or been fluid. Korea once went through a "Three Kingdoms" period with the Kingdoms of Shilla and Paekje in the south (and the Gaya confederacy, but no one really talks about it) and the Kingdom of Goguryeo in the north. The three were constantly at war, ending with Shilla dominating the others.
What was the capital?
It's changed quite a bit. Seoul is a relatively new name. It was known as "Hanyang" under the Yi Dynasty. Before that under the Kim Dynasty of Goryeo, it was Ganghwa (mainly to avoid falling into Mongol hands) though it had changed before that. Shilla's capital was Gyeongju.
What are some important historical milestones and landmarks of the country?
What caused the rift?
Between north and south? Generally? Geography. Every country has north-south differences. Korea traditionally had eight provinces, each with their own distinct culture and accent, and only because of the most recent division and the economic isolation on either side of the border has that cultural divide widened.