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Road surfaces and conditions in Thailand vary from perfect smooth asphalt to the most muddy,slippery, potholed roads you can imagine. The main problem you will have in driving is that the condition of the road you are on can change with little or no notice of what's coming up. You might be cruising at 120 km/h on a beautiful, smooth, new hiway, and the next thing you know, you're flying out of your seat while your bike is diving into a hole. You must be constantly aware of what's ahead. Holes and dips can be difficult to foresee, so watch the traffic in front of you. If you see brakelights or a car bouncing or swerving, slow down and prepare for it. Don't 'run-up on traffic'. Give yourself plenty of time and distance to assess the change ahead. Look for changes in the road color, signifying patches or holes. Be careful of patches because if they are new, they will usually be higher than the road surface surrounding it. If the patch is asphalt (black), it may be rippled or collapsed again, so be especially careful with them.
Bridges are another hazard. With numerous irrigation canals throughout Thailand, you will come across many bridges, most of which will cause you to gain altitude when getting on the bridge and lose altitude when getting off. Unless you are sure of you and your bike's ability to fly, adjust your speed accordingly.
On many roads the outer-left section, which in most countries would be designated an emergency lane, is where motorcycles are required by law to drive. Since this lane doesn't usually get much heavy road traffic, it can be the smoothest part of the road, but also the most dangerous for many reasons. For example, when large vehicles breakdown, they park in the bike lane. While parked, they will usually set out some rocks or tree branches a few meters behind the vehicle as a warning to on-coming vehicles at night. Unfortunately, when they leave, they rarely remove the warnings, leaving a dangerous obstacle in the road. I had a friend who was seriously injured by hitting such an obstacle at night. Also, this lane is used by people walking, cars waiting to enter or cross the road, or motorcycles going in the wrong direction. Personally, I avoid this lane except when the main road is just too bumpy or when oncoming traffic is passing.
In the mountains, are some of the roads that bikers love. Banked curves are what many of us dream about. But 
sometimes dreams can become a nightmare when that beautiful banked curve that just asks for speed is covered in sand or rocks. On asphalt roads, many banked corners become rippled in the heat or oily from the separation of asphalt chemicals. Also, be very cautious of oncoming vehicles 'cutting' the corner or passing blindly. When coming down mountains, be aware of overusing brakes. Use engine compression as much as possible to avoid your brake fluid over-heating and the subsequent loss of your brakes.
If your bike is a rental, call the shop you rented it from and they should send someone to come and get you.
Before you commit yourself to a bike, check it out thoroughly first. Look to see if it is new or not. Make sure it has license plates (the cops may 'request' 500 baht or even take your bike if you don't have one). Check the tires, all lights, and horn before riding. If anything is not working, make sure they fix it before you take it, or choose another bike. Take it for a short test-drive and test the brakes and handling. If anything feels weird, don't take the bike. Make sure you know where any fluids are supposed to go and what fluids are required. If a bike gets damaged because you didn't add fluids or put in the wrong type, you could be held responsible for repairs.
Going up the cost ladder, some shops have been known to steal back their own bikes from your guesthouse or hotel and then charge you to replace the bike. This would cost you a minimum of 40,000 baht for a Honda Dream and much more for a bigger bike. A possible protection would be to buy your own chain and lock for 200 baht ($5) and chain your bike (rear wheel and frame) to an immovable object.
Lastly, and most costly, but thankfully, extremely rare anymore, is the shop that plants a bit of dope on your bike and then sends a friendly cop out to your guesthouse or hotel for a 'routine inspection'. Finding the dope, you will be offered two choices: go to jail or pay-up. Obviously, they want your money more than taking you to jail, so you will end-up paying until they are convinced you are dry. This will include your jewelry and everything else of value you have.
Like I said, renting a bike will probably be hassle-free, but let the renter beware...