How long have you got? Most of us with years of riding experience could write a book on this subject. The brief version;
Helmet. Get one that fits! Fit is the first and most important consideration. Certain heads suit certain makes better than others. Try loads on a leave them on for a few minutes. A plain colour will be cheaper than a race replica design. You are better of getting a plain helmet that is further up the manufacturers range than you are with a more basic model with Rossi paint job. Personally I avoid lids with lots of 'furniture' on them like spoilers and fancy vents. The benefit of these is limited and the more you have stuck on the outside of the shell the nosier the lid will be at speed. (Maybe not a huge consideration on a 125)
If you are starting out I will assume that you will only have once set of kit to start with. In which case you want fabric jacket and trousers. Cordura is considered the holy grail of waterproofing and breathability but it is expensive and other manufacturers material are fast catching up. Any of the big brands should offer decent quality with the exception, in my opinion, of Frank Thomas as I have always been disappointed in their stuff. Again, fit is important. You don't want to be distracted by uncomfortable clothing when you should be concentrating on your ride. In terms of keeping costs down, last seasons kit is no less effective that this years but can often be had a knockdown prices.
My final offering - wear earplugs. You will be extremely grateful you did when you are older - particularly if you take to biking and ride for many years.