Help with hirigana

I have been trying to teach myself Japanese and have been coming across words that i find it hard to translate romanji words into hiragana. There is no character for shu, but the word for weekend is shuumatsu. Also, I have problems with a random k. LIke in kakkoii. Also mottainai is a another culprit of this. Is there a hard fast rule for this? Or does it depend on the word? Am i learning wrong?

You have missed something very critical somewhere. You shouldn't be learning in romaji if you can help it at all. If you're at a point where you're learning words, you should at least understand things like using a small つ or a small ゆ. Mottainai is もったいない and shuumatsu is しゅうまつ. Also, it would be best to ask questions like this in the Japanese thread already on /lang/.

Yeah you're learning wrong. I hope you're not using a romaji textbook.

If you want a guide to follow, Tae Kim's is a pretty good start http://www.guidetojapanese.org/learn/

Local Japanese thread is here https://dis.4chan.org/read/lang/1330084899/