New owner of '99 gz250. Lots to learn.

[ThisGuysFirstGZ]

Hey, guys I just purchased my first bike and it is a gz250 that runs and rides and brakes and all that. I have never worked on a bike before this. However, I noticed a few minor things that could lead to big things after research and am looking for help. Before I break it down, let me give you a limited history:

Bike was purchased from a guy who did not ride it often and re-worked the wiring himself, along with some minor things like throttle set-up, and some cosmetic things (seat, tail light, took off neutral indicator and turn indicators. It's got 3600 miles on it so it's "young"

Now for the breakdown:

1)The bike starts on "on" fuel position with full choke, but not always. Starts on prime right away.

2)Switch it back to "on" after starting and it will sputter out after riding for aprox 10-15 seconds. Rides on prime to go home (just around the block).

3)Noticed oil level is very full. Checked oil cap and smells like gas. Air filter smells like gas too. (normal I think, but looks like it's wet with gas)

I did a lot of youtubing for the answers and some were addressed, but here I seek the sage knowledge of other gz250 owners for the best solution. Should I have a mechanic look at it? Is this a job that I can do myself? Is it just a matter of replacing the petcock and changing oil? Or do I need to do something bigger like rebuilding the carb? If this is in the wrong thread, please redirect me.

Thanks for your time from a humble new beginner.