For a first try, its a good start. To help with the "constructive" criticism...the choppy edges can be resolved by adding a feather of about .3 to you selections. Not sure how you make you selections, but when I do them using the pen tool, there is an option after you choose to "make selection" for inputting the feather. That will help a lot. For the best result you should start with a base that has at least 1600x1200 resolution. As for the interior, your seats are a bit bright...the contrast seems ok because it does look like they are behind glass, but the darkness of the seats doesnt match the existing interior around them. Also, they seem a bit large for the car. As for the wheels, using the same wheel 2x is a no no. Looking closely you see that the rear looks flat after distorting it to match the shape it should be. I've done this before and fixed it, but that requires a bit of brushing...when I was done it was pretty much a brand new wheel.
All of the mistakes are very common for beginners so no worries. You should
some tutorials and you will learn some nifty tricks (in fact I think there are many hosted on this forum).
Oh lastly, when you make a 2 door conversion its best to extend the door line so the proportions of the car look right. As it the back window is much too large.
Keep it up, you will get there...Everybody started where you're at now.