I loved my CBR 125 - cracking little bike. Few tips:
1) keep the gear linkage well oiled. Either grease it reallllly well every month or (my preferred option) WD40 it quickly every few days. The latter is quicker and provides more consistency but it depends on if they've greased it well already as WD will try to displace grease.
2) Keep an eye on the chain. The stock chain is crap and replacement ones are usually crapper. I say crap meaning they stretch really easily. You do NOT want your chain coming off. It can be very painful. If you've not already got one, you'll need a torque wrench, two spanners (8 and 10 I think but check the manual, don't use adjustable ones, they just ruin nuts) and the appropriate size socket. Check it every week for tension as per the manual and lube it regularly or it'll chew through chains. DID do a replacement chain that lasts a lot longer but my gearbox started making a funny noise shortly after it went on and eventually broke. Not sure if they damaged it during fitting by torquing the axle nut whilst not properly engaged in gear or if the chain put extra stress on somehow. I'd doubt it was the chain itself but rather the fitting.
3) They're very sensitive to tyre pressures, especially the front. Check those every week as well. You'll be able to feel when they get low with experience.
4) The clutch cable needs oiling occasionally - about once a month if you use it in winter. Lazy man's way is to use a thin oil and squirt it down whilst pumping on the clutch but there's probs a better way.
5) If you're reaaaally light, try shoving a chain under the pillion seat to settle the rear end down a bit. Don't carry a chain in a backpack as it'll wrench your back round if you come off.
6) Try decent petrol in it - it responded well in terms of MPG to 99 octane instead of 95. Try it for yourself and see how well you do and work out if the extra mpg or convenience is worth the extra cash. I got just enough extra out of it to mean I could fill up at weekends rather than on a Friday.
7) Look at a replacement horn. They're cack. I got one from Acumen Electronics which was cheap but only just adequate.
and finally 8) Always chain it up (at least an Oxford Hardcore chain and lock, ideally better, just make sure it fits under the pillion seat) and if you can, get a cheap alarm. Mine was £15 and was just a tilt switch, a key switch and a batter in a box. Not much, but enough to deter kids who decided to sit on it and play and draw attention. Big boy alarms may flatten the battery and there's precious little room to fit them. If you don't chain it up with something chunky, it WILL get stolen. Kids have big bolt cutters these days.