You'll need to know more than that.
Is it 2-stroke or 4-stroke (usually says 2T or 4T on the bottle).
Also, what kind of oil, as in there'll be a number like 10-W-40, 10-W-50 or 15-W-40 on the bottle.
For future reference, when you come to change the oil, I understand 10-W-50 is a good one for the DT125.
Other owners will probably explain further/better than me.
Possibly.
Let's sort the oil side out first. Then we'll look at the other things.
Short version - Read all the way before attempting:
You will need - Socket set and ratchets, or a set of long spanners. A wide and deep pan, old oven tray or some other open-topped 5 litre container. Rubber gloves. A funnel, normal or improvised (like the top half of a 500ml plastic Coke bottle). A friend who will not drop your bike.
Run bike for 5-8 minutes in neutral.
Stop bike and take key out.
Get bike on level ground.
Have someone hold bike upright, unless you have a bike jack.
Place pan under bike engine.
Remove oil filler cap.
Loosen off the sump plug (drain screw), but pay attention to how tight this is. You'll later do it up to the same tightness.
You *might* want to wear rubber gloves for this next bit... If nothing else, grab a free pair of those plasticky things from a petrol pump.
Slowly remove sump plug and drain some oil, as described by Airhead above.
Be careful, the oil will be very warm at least, if not actually HOT!!
The oil will flow out very quickly. Don't drop the screw into the pan either, as it will be slippery.
Let some oil out.
Put plug back in just enough to stop the flow, then check level (see below) in the sight window, which is probably on the left side.
If it's too low, put some back in. Use a funnel to avoid spillage.
Repeat until you're sorted.
Put the sump plug back in and tighten up nicely, but NOT too much. Just about where it was last time.
Make sure you put the sump plug back in.
Now put the filler cap back on.
Now check you have put both the filler cap and the sump plug back on.
And check again, to make bloody sure.
This is a different bike, but this is what you're looking for, level-wise... There may be only lines, rather than an F and L, or no marks at all, but halfway to three-quarters, with the bike held upright is what you're looking for: