that's a pretty broad request.
as a newish owner, I'd say the first change is the carb. do a search here, there's a bunch of references to ones on ebay that should guide you. also get a 30 pilot and 115 main jet to fit the new carb. the jets are pretty common and you should be able to find them at your local bike shop if you don't want to wait for them online. settings for the pilot screw should be 1.5 turns out. Needle should be in the second clip to start.
second change I did was the 17 tooth sprocket on the front. when you do this change, there is a small tab that tucks in behind the starter on the sprocket cover. that has to be removed in order to get the cover back on. the issue is there is just not enough room with the larger sprocket to get that little tab back behind the starter. it's easy to remove, just use anything you got. I used a grinder because I have one, but a hack saw or anything else handy will do the job. it doesn't have to be real pretty. it's hidden from sight.
Next change is the 45 tooth rear sprocket. You may not want to do this if you plan to ride primarily off-road. I am doing it because I want more street speed and won't be doing heavy trail riding. of course, you can always swap them around based on the type of riding you plan to do at the time.
Regarding the bike in general when you receive it, first thing you should do is change the motor oil and fork oil with some reliable brands. the stuff from china isn't very good quality.
Some have also recommended changing the plug for a good quality brand name to get better hot spark. I have not done this yet.
Other than that, I'm really happy with the bike now that it's all sorted out. I just ride it like I stole it and enjoy!