Some basic questions about Japanese

I'm currently learning japanese and I've got some questions about the language:

1) I've read that there are two type of readings to kanji: onyoumi - chinese when the kanji comes with another kanji in the same word. konyoumi - japanese when the kanji is the only one in the word. But when i started to learn kanji i've seen the kanji for "month". It said that the on reading is getsu and the kun readin is tsuki. But in their examples they say that in the words this month you use getsu (kongetsu) and in the words every month you use tsuki (maitsuki).

my question is: In both examples there's another kanji along with the "month" kanji so why are the reading differences?
the site with the examples: How to write moon in Japanese - moon in Japanese kanji

2)The words in japanese text have no spaces between them so when i get to a word i don't know i also don't know where that word ends so even if i know the words afterwards i'll miss it because i won't know where they'll begin....
In order to read japanese text you have to know all the words in the language?? doesn't make sense to me, so how can you read a text?

3)Last one, In the sites i've learnt from i've see some suffix in the japanese grammar such as: masu,desu,deshita,tai,mashou,kunai,katta and etc...
But I've heard some words with other suffix like: ru (taberu), nasai, te, ro (yamenasai, yamete, yamero), ne (shine),de(shinde). when do i use these suffix? and what other suffix are there in japanese?

1) The "on'yomi in compounds, kun'yomi when it's by itself" guideline is a guideline, not a rule. It's also perfectly possible to have readings which are mixed on and kun, and readings which are neither. Many of these exceptions are common words (for example 今日 for today is きょう). You know the reading by knowing the word - although given an unfamiliar compound with familiar kanji guessing based on on'yomi will work a lot of the time.

2) You don't have to know every word - and actually for fluent reading you shouldn't have to. Parsing a sentence can be done based on grammar, although the more vocab you know the easier it gets. This is actually harder with all-kana text (like books for very young children), so it's not uncommon to see all-kana text with spaces in. Adding kanji makes it much easier.

Basically with some experience you can recognise things as nouns, adjectives, verbs, etc. (and in what form they are), even if you're not sure of the meaning.

3) Words conjugate (change form) generally from the end. So "taberu" (dictionary form of the verb "eat") always keeps "tabe" but the ending changes - tabemasu, tabenasai, tabete, etc. These aren't generally called suffixes, because depending on the verb the stem changes as well - e.g. "nomu" is "to drink" and the matching forms are nomimasu, nominasai, and nonde.

Don't overwhelm yourself by getting a list of conjugations and trying to learn it all at once. You could probably write a book, or at least a chapter or two, just on uses of the te form.