You might wanna start
here and give this a quick read before moving on.
Next, you will want to check what size sprockets are currently on your bike. The stock sizes are; 14 tooth front and 45 tooth rear (in the US, different countries have different stock sizes). Rear sprockets normally have a number stamped right on it to make it easy to identify but you just may have to count teeth for the front.
Changing the sprocket sizes (# of teeth) can affect how the bike feels, so if you don't want to change that, buy what you currently have. It is common for riders to install a 15 tooth front sprocket to give the 250 a "calmer" feel at highway speeds. It's a personal preference.
For normal road use, good price without overbuying and the longest life possible, you should be looking for steel sprockets and an O-ring/X-ring chain. You can buy the parts individually or in kits. I have found that buying the parts individually can be a cheaper option if you are a smart & savvy shopper, but the kits make it super easy.
Sprockets
For the price and quality, it's pretty hard to beat
JT sprockets for the sprockets but I don't use their chains so I have no experience there. I also like Vortex sprockets and their prices are finally becoming more realistic.
Chain
I commonly use D.I.D (expensive), Renthal & EK (cheaper but fine quality) chains. Your looking for this size 520 w/ 106 links. They are commonly sold with more links than what you need and require the installer to cut the chain to length and install the master link. It's easier than it sounds but does need a chain breaking tool or a steady hand with a
Dremel and small cutting wheel.
Brakes
You should be able to find an average set of rear brake pads for around $30 or less. EBC or an equivalent brand should do just fine for normal everyday riding.
Here is a common price breakdown (USD) prices vary from website to website;
Front sprocket: $15-$25
Rear sprocket: $30-$50
Rear brake pads: $30-$40
520 106 Chain: $100ish
Also, you might find that ordering from your dealer can save you shipping time/cost. Plus it's another set of eyes to double check that your ordering the right parts or haven't forgotten anything. I have no problem paying a few dollars more to have someone else handle that for me.
Happy shopping!
