Are you handy? oOr do you have to get someone to fix things?

I’ve assembled the most of my furniture in my room, and I hooked up my computer, but I’m not good at sewing.

most = talking about quantity, amounts
the most = superlative comparison (the most funny -> the funniest, the most pretty -> the prettiest， etc.)

I can concentrate on something which is in mys I'm interested in, like oil painting, but I wouldtend to be lazy in areas where I’m not interested inother areas.

You can use "things I'm interested in" to explain it more concisely. Similar examples: things I like, things I usually eat, things I think about, things I'm not good at, etc.
And "other areas" is neater to use here because you already introduced the topic (things you're interested in) earlier in the same sentence.

I’ve assembled the most of my furniture in my room, and I hookedset up my computer, but I’m not good at sewing.

You dont need the because it is not one specific furniture since you say most. Hook up does mean to provide electricity to equipment but I dont think it is the best word choice here.

I can concentrate on something which is in mythat I am interested in like oil painting, but I would be lazy inam lazy when it comes to areas wherethat I’m not interested in.

I understand what you want to say but it is more clear this way and with correct grammar. To be interested in something is a very useful phrase.