So, very rough rule of thumb is that when a kanji is sitting on its own, or has okurigana on the back (tacked-on hiragana used to indicate verb and adjective conjugations) then you use the kun’yomi, while if it’s in a compound word with other kanji, you use the on’yomi. This is a very rough rule, though, and there’s about a thousand and one exceptions.

In this case, when 力 stands alone, it’s ちから. For example 私の力は十分じゃない = わたし　の　ちから　は　じゅうぶん　じゃない

When you use 力 in compound words, it’s りょく　or りき. Poking through the dictionary, looks like it tends to be りき when it’s at the start of a word, and りょく when it’s at the end. For example:
協力 (cooperation) = きょうりょく
原子力 (atomic power) = げんしりょく
力説 (emphasis) = りきせつ
力学 (mechanics) = りきがく

To toss in an exception:
力水 (water given to sumo wrestlers before a match – literally “power water”) = ちからみず

I’m glad someone asked this because I have gradually been getting more and more confused about some of the readings and their relevance.

However thanks to you two I think I’m one step closer to understanding kanji xD

The relevance is something like this: When kanji was originally introduced from China, Japan already had its own spoken language. The Chinese characters were thus used to write Japanese words which already existed, so the Japanese reading was given to the kanji – this is the kun’yomi. However, the Chinese characters already had readings attached to them – namely, the Chinese readings – and these were brought over too, becoming the on’yomi.

In my experience it seems to me that the more nature-themed and concrete a word is, the more likely it is to use the kun’yomi. So, for example, 川口 (river mouth) uses the kun’yomi, かわぐち, because river mouths exist basically everywhere in Japan, while 人口 (population) uses the on’yomi, じんこう, because the ideas of population-counting and census-taking and whatnot was introduced from China.

You’ll tend to find the kanji dictionaries give the kun’yomi in hiragana and the on’yomi in katakana (because it’s a “foreign” introduction), but this is purely to differentiate the two – in regular usage, readings are usually given in hiragana.

Almost all kanji have both a kun’yomi and an on’yomi. Often multiple on’yomi. Some kanji have no kun’yomi, and there’s even a handful with no on’yomi.

After all that, your best bet is probably to ignore the fuss over “well, this is kun’yomi and this is on’yomi” because you’re only going to forget. Instead, learn the vocab, and you’ll pick up the readings of the kanji as a result. Vocab is basically always read the same way, though there are some exceptions (rule number one in Japanese: all rules have exceptions, including this one). Once you’ve got a bit of vocab under your belt, you’ll start to get a feel for which reading goes where, and you’ll even start to be able to intuit the readings (and meanings) of words you’ve never seen before – and even kanji you’ve never seen before.