go carefully and remember that a motorcycle tyre has to provide its grip using two contact patches, one the size of a postage stamp and the other about as big as your palm.
Cars do it with four patches the size of books so just don't take the piss with the throttle and round corners. Car drivers will try and knock you off. They ignore the potential consequences and try and kill you through stupidity and through incompetence. Fit your bike with a big horn - Stebel Magnum is a good choice as it just swaps out with the original with no extra wiring. Also get replacement high power bulbs. Check the size and go and find some that are the same wattage but put out extra light. Don't go for higher wattage as you can melt the reflector.
Ride cautiously and refrain from filtering until you've got decent control over the bike. Get practising emergency stops.
Consider getting a hivi as well. There's debate over whether it helps but I can tell you that out of a group of cyclists, I notice the one in the hivi a lot further off.
Also, do not EVER EVER EVER EVER ride in the gutter or move out of the way so people can go past. You want to be in the centre of your lane and do not yield. That extra space gives you something to move into for when morons overtake you really close or overtake coming the other way into your path.
A lot of new riders neglect their chains. Find out how to lube it and adjust it. I've seen them come off before and the rider was lucky it didn't take his ankle off or wrap around itself. It's potentially fatal if it comes off at speed or round a bend.
Read through the riding skills posts. There are some stupid questions and stupid answers in there but there's a lot of gold that is real world advice that'll stand you in good stead.
Oh and a big bike is good in some ways but you'll never get the fun of thrashing full throttle all over the place and trying to make everything flow sweetly to get the best speed that you get with a 125. Something about a big engine spoils the planning and strategy that comes with getting a 125 through traffic.
Drug fuelled ramble over.