Ooh, boy, it's been a while since there's been a question I could answer on here!

So, for some context: prior to the end of WWII, Korea was occupied by Imperial Japan. Kim, as a younger man, fled to Manchuria, and he later joined the Communist Party of China and was active in anti-Japanese actions. In 1940, went to a military camp in Russia, where his son, Yuri Irsenovich Kim, was born. You might know this son better by the name he adopted later in life: Kim Jong-Il.

Kim Il-Sung served in the Soviet Army until the end of WWII.

Beria, head of Stalin's secret police, recommended Kim be put in charge of the Soviet-occupied sections of Korea. He hadn't been in the country for over two decades, and he struggled to speak Korean: essentially, he was appointed by the Soviets. He began to solidify power by recruiting and training military support. Technically, in 1948, the Soviets held elections, but they were of dubious credibility. The Communists in North Korea continued to consolidate their rule, and Kim began to have a cult of personality form around him.

Some of his earlier policies were actually popular: redistribution of land, the nationalization of healthcare, and workdays were shortened.

This, of course, soon led to the Korean War, which began in 1950 when Kim and his forces crossed the 38th Parallel, supported by the USSR and China (which had only recently ended its own civil war and been consolidated under the Communist Party).

After the Korean War, the country was split into North and South Korea; as Cha discusses in The Impossible State, during these early years, quite a few people actually fled the South to go to the North. This was in part because of the dictatorial government in South Korea, which was violent and chaotic, and the huge amounts of aid North Korea was getting from China and the USSR, allowing them to rebuild their industry, which had been devastated during WWII. I'm a bit fuzzier on this, but if memory serves, South Korea had been more treated as an agricultural territory under Japanese rule, which also contributed to setting them back.

North Korea continued to remain more dependent on outside aid like this, though as time went on and disagreements arose between China and Russia, Kim would play different sides. After Khrushchev gave his speech denouncing Stalinism, for instance, Kim turned to China more heavily for aid; he feared that a similar dismantling of his cult of personality might happen after his death. Due to this aid, which kept the country going for quite a while, as well as his cult of personality, he was considered quite popular; Kim Jong-Il, on the other hand, was viewed as a bit of a failure within the country, if memory serves, I believe that's from Fisher's book.

So, in essence: Kim rose to power because of the Soviets, who appointed him to his position and kept him in power along with China. He was popular, at least at first, due to some reforms, and the heavy pushing of a cult of personality, which was meant to overcome the fact that he hadn't been in Korea for over two decades when he returned to take power.

Sources:

A Kim Jong-Il Production: Paul Fisher

The Impossible State: Victor Cha

Dear Leader: Jang Jin-Sung

The Aquariums of Pyongyang: Kang Chol-Hwan

There's another book I swear I referenced, but I'm drawing a blank at the moment.

Apologies if any of this is vague or unclear, feel free to ask for clarification on anything; been a long day at work and I'm a bit exhausted.