I just went along with my class, but we had people of varying skills. If you find a particular grammar point to be really easy, I wouldn't spend as much time on it.
Thing about genitive case: none of my class actually learned it until year 12 because we had a new teacher. In fact, it's barely necessary for writing in your exams anyway and you can use alternatives a lot of the time instead.
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OK, I have a better idea of where you're at now anyway. I think the number 1 best thing you can do with your time is understanding these three main things:
1. nom / accusative / dative case, when each is used, and why
2. adjectival endings
3. pronouns and prepositions
Like me, you aren't doing English Language. In my cohort of 6 people, literally everyone was doing English Language except for me. As a result, getting your head around the technical lingo jingo might take a bit longer. I honestly didn't waste my time trying to learn the linguistic aspects of every rule, but just learned how to apply it like an ordinary language.
Anyway, I'll try to summarise those three above topics into the simplest form I can for you. You said you already understand these, so below is just a quick summary to confirm your knowledge.
Spoiler
First up, each of these cases will have an effect on the articles before words. You will probably already know that if a dog is the subject of a sentence (it is the thing doing the verb) then you will use 'der Hund'. Conversely, if the dog is getting affected by the subject (and is therefore the object, getting verb'd), then it will be 'den Hund'.
Subject = blue, direct object = red, indirect object = purple
1. Der Hund ist braun. (The dog is brown)
2. Der Mann streichelt den Hund. (The man pats the dog)
You should already be familiar with this. If you can follow this, you're already 2/3 of the way there. One minor rule to note is that if you're using the verb 'sein' (to be) then you're saying the subject is EQUAL to something else. If this is the case, that means the something else is also the subject, and therefore in nominative too.
E.g. Der Mann ist der Sohn von Markus. (The man is the son of Markus)
Let me know if you don't know what I mean here.
Anyway, to get back on topic, the only thing left to understand is dative. The dative case is simply used when you have an indirect object. This is when you start using dem (masc/neut) or der, for example.
1. Der Mann gibt dem Hund einen Knochen. (The man gives the dog a bone)
We've got a bit going on here. The easy thing to do is just identify what the subject is and what the direct object is. The subject in this case is the man, because the man is giving. Then we ask 'what is getting given?' and the simple answer is clearly the bone. This leaves one thing left: the dog. Therefore, the dog is the indirect object. This is because it is an object, but not immediately affected by the subject.
Something may also become accusative or dative depending on what preposition is used and the context in which said preposition is used. Examples of prepositions include aus, bei, mit, in, ohne, durch, etc. You will have already or will go over this in class in the future. If not, tons of websites have awesome explanations and I can give you a hand as well.
How much do you know about adjectival endings? If you've perfected them, then splendid. You will be very happy to know these in the future so you don't have to check every time. I'm pretty sure we had already covered most to do with adjectival endings at that point.
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Genitive is a case you've probably noticed in charts/tables as well. Essentially, I consider it the case of 'possession'. I'm not very good at providing technical explanations when it comes to LOTEs, but hopefully you'll soon understand what I mean by this.
Technically, there are also prepositions that force the genitive case, but this is really extension work and you'll only really need to be able to understand it rather than formulate it yourself.
To start off, you can use the genitive to show possession in a manner that the English words 'of the' normally would.
I'll provide some examples so that I can explain it a bit more clearly.
1. Das Baby der Frau heisst Alex. (The baby of the woman is named Alex)
2. Die Katze des Mannes scheint krank. (The cat OF THE man seems ill)
The simpler way of doing this is just using 'von' (of) in which case you just use the dative instead. Anyway, if you want to do it this way, you're unfortunately just going to have to learn the table. For the definite article (the), masculine and neuter go to 'des'. You might also get an -es ending on the word depending on the amount of syllables (e.g. Mannes). Feminine and plural go to 'der'. Similarly, for the indefinite article, you start following stuff like 'Der Mann
einer Frau' (the man OF A woman). You probably get the jist, it's just like relearning the original articles.
Here's a website for some legit explanations:
http://www.rocketlanguages.com/german/learn/german-genitive/My explanation is a bit dodgy as well because I can't find my German notes book. I'd recommend asking your teacher to spend a lesson on it if you're curious to learn more. Bottom line: if you can't FULLY use it, don't worry. You've mainly got to be able to understand it when reading or listening.
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In terms of more stuff you can learn, I would then go on to learn the following concepts after you have nailed the above in order to make your writing look more impressive and more "German". If you skimmed all of the above stuff thinking 'I actually already know this stuff, I just struggle with harder concepts' then consider these as well:
1. Relative sentences in the nominative case
2. Passive sentences
3. The subjunctive (Konjunktiv II)
How well acquainted are you with the above? Have you completed future tense as well?