M'athchomaroon, zhey KhalToddy.
The somewhat complex declension and conjugation systems for Dothraki can be intimidating, especially for someone who has no experience with foreign languages. I was almost in that category when I started working with Dothraki.
One really important thing that is not obvious to a beginning user is the words given in the dictionary or the wiki are not in their most basic form, especially for nouns and verbs. You've picked up on some of this already, but basically, nouns are given in their nominative case and verbs in the infinitive (i.e. to verb) What does this mean?
Dothraki uses cases on nouns to indicate their function in the sentence: subject, object, 'possesive', or if they are indicating the action is moving towards or away from the subject. English is different. it uses word order to indicate a noun's function in a sentence.
Very carefully study the noun declension charts in the grammar section of the wiki. You will notice that the basic 'stem' form of the word is used when it is the object or accusative case. So, unless the noun is the object of the sentence, it will always have one of the suffixes attached to it. Although an effort has been made to explain things simply, the terminology takes some getting used to. If you don't understand a term, look it up online. Wikipedia is your friend here.
Verbs are a bit more complex. The infinitive form of a verb, as given in the dictionary, is not the most useful form of a verb, as most sentences do not need the extra 'to' in them. The only tense without any prefixes or suffixes is past tense. There is also a suffix difference, if the verb is used in a positive or negative sense. So, a 'stem noun' has no suffixes, and is past tense, indicative form (opposite of infinitive), and positive grade. Everything else needs suffixes, and in some cases prefixes. Again, study the conjugation chart very carefully!
On many words, especially nouns, it is hard to determine if the correct nominative ending is -at or -lat. On words that have a misleading ending, look in the wiki. Most words that have a misleading ending have their stem form shown, so you can learn what the correct endings are, that you can use.
Don't be afraid to ask questions. there are several folks who watch this list and are always glad to help!