Assuming that the "four wheelers" you have ridden were sport models with clutch and standard gearbox arrangement then you should at least have the basics down for operating a bike. The transition to a twist grip throttle instead of the thumb throttle might take a little getting used to but I would expect you would get the hang of it fairly quickly.
For your size and weight you are going to need a full size bike, either a 250 two stroke or a 450 four stroke.
Let's talk about maintenance for a bit: Bikes that have high performance engines (MX bikes) need frequent maintenance, and it doesn't matter if it is a two stroke or a four stroke. A two stroke, however, is much easier to work on and there is less to do. It is common for a MX bike to require a top end rebuild after 20 hours of operation. A top end rebuild on a two stroke can be a new piston and gasket set, which is ~$200 and a few hours of work. A top end rebuild on a four stroke will often include head/valve work, which can end up costing a lot more and just the time for the tear down and reassembly will be much greater.
"Trail" or "off road" bikes will typically have "low performance" engines. They have lower compression so they can operate on the cheaper pump gas and all the engine components are generally heavier, beefier so they last a lot longer. Keep the oil changed and the valves adjusted and these bikes will run forever. These bikes are heavier, cheaper, lower seat height and generally have limited suspension travel. They are very good for a beginner rider, and reasonably good for general trail riding but are not adequate for a MX track.
The cheapest entry in dirt bikes for you would be a 250 two stroke, about 10 years old. There are lot of these bikes out there and most of them still have a lot of life left in them. You should be able to pick one up for around $1500 in reasonable shape. Look for a Yamaha YZ-250 or a Honda CR250 or similar bikes.
Assuming that the ATVs you have ridden were all four strokes you may have a bit of difficulty learning to tame a two stroke. A two stroke is capable of delivering almost twice the power of a four stroke of equivalent size, but the power can be difficult to manage. A four stroke, on the other hand, is much more forgiving and easier to deal with.
If you would rather stick with a four stroke then you can look for a Honda CRF450X/CRF450R or Yamaha WR-450 / YZ-450. The difference between the bikes listed (separated by / ) is that the first bike is more of a trail bike built on a race bike frame and the second is a full MX bike. The trail bike versions have electric start, headlight, kickstand and wider ratio gear box. The bikes listed are all around $7K to $8k new.
If you wanted to get on something right away and plan on upgrading in a couple of years then you might want to look at a Yamaha TTR-230 or a Honda CRF230F. These are the trail bikes I mentioned earlier, which would be great for you to learn on but you will grow tired of them fairly quickly.
Rod