Post some surrounding information, or even better the design brief together with your work, it's very difficult to receive useful feedback without this information as the work has no context. The more information you can provide the better your feedback is likely to be.
1. Front tyre sinks in and doesnt line up with reflection
2. You can see reflections through the windows that dont match the entire piece
3. Adverts would never use an actual number plate
4. Cutting around the car is poor
5. There is no definition between sky and water(not sure what it is)
6. Cut out around window wipers is messy
To be honest id try out some PS tutorials and have a practise. Also maybe compare it to an actual commercial advert?
Mimoza, do you mind if I ask what the assessment is for? Also, what software are you using to create these images?
You might find inspiration from studying current car adverts and their composition, as well as paying attention to how car manufacturers choose to portray their cars. i.e. An all-terrain vehicle would be best shown tackling different off-road environments, but a sporty car would have a more modern, slick presentation. Try to show the car you want to advertise in context rather than a generic background. Even if you just use simple graphics to "suggest" it instead of photographic backdrops.
Why don't you just take a nice car to a nice location, take pictures of it & just re-touch it in Photoshop?
Just Google images: car ads. You'll notice that a lot of the ads are just pretty photos with a bit of bold text & a lot of smaller text (car specifications). You might want to give that a go.
It seems you are a novice at Photoshop so I suggest you browse a LOT of tutorials and inspiration sites. And keep looking at other car ads for an idea as what-to-do.
Or else just hang around DF :lovedf: