This morning, I went to the Volkswagen "4 sS store" to maintaihave maintenance done on my car.

I have no idea what a 4 S store is, although I found some references to it on the internet. So I leave it, but it's often helpful to put an unusual term like that in quotes - it means "yes, I am saying exactly this, I know it's unusual but it's not an error".

I chose "have maintenance done" because of the shared root with "maintain". Maintain feels strange here but maintenance is normal. Don't ask me why :) You could also say "have my car serviced", "service my car", "have some work done on it", "have it fixed", "have it seen to", "have it inspected". Note that in most of these you aren't the person doing the actual verb, because you're going to ask the repairmen to do it. But "service" works.

ThisIt's a Passat, which was bought in 2009 and it was also the first car in my life.

Slightly unnatural to introduce "this Passat" here. It implies there's already a Passat, but the fact that it's a Passat is new information. You could say "This car, a Passat, was bought". My suggestion is more natural than that.

Not a big deal though.

For some reason, I have driven it again since a couple months ago and I feel it was in bad condition compared to what it was before.

Couple of options here.
"I have driven it" is fine if the amount of driving you've done isn't important. At one time, at many times, repeatedly - it doesn't matter. At some time, you drove it, that's the only thing you want to say. But then including ongoing time frame "since a couple months ago" is strange. Better: "I have driven it again in the past couple months".
Maybe "I have been driving it again since a couple months ago, and I've felt..." - focusing on you driving it as an ongoing action. Not necessarily all the time, but regularly.

"For some reason" implies that you don't know the exact reason. Not that it isn't important, or that you don't want to explain - you don't know. So if you don't that's fine :) But I thought I'd mention it.

After the overall check, the repairman told me that a lot of problems have taken place in the carthe car has developed a lot of problems.

I don't like "a problem has taken place". I would prefer "a problem has occurred". But that's still quite unnatural. I would rather have the car as subject and the problem as object. There may be a nicer word than "developed" here but I can't think of it!

If I solvehad them all fixed, it would cost about 6 thousand yuan.

Mechanical and physical problems are generally fixed, not solved. Solving is perhaps more about logic, abstract problems. And again you are not the person that's fixing them, you're asking someone else to do that, so you "have them fixed".

I only had them fixed a few necessary things that cost more than one thousand yuan in total.

Here we have "fix" already! If you want to avoid repetition, "I only had them do a few ..." works.

I also let a salesman evaluate the current pricvalue of the car, disappointingly it didwasn't costworth much.

"Cost" doesn't work well here for several reasons, I'm not sure I can adequately explain. It's more often something you talk about if you're the buyer, or if you're talking to someone who might be buying. And with it in the past - "it did not cost much" - it sounds like that price is in some sense *completed*. When I say "It didn't cost much", I mean "I bought it, and its price was low" or I mean "I could have bought it, and its price was low, but I didn't". Here, you still have the car. So to use cost you would probably say "it *would not* cost much" - if you were to sell it, the price would be low.

It's a lot more natural to talk about its value and how much it is worth.