If you are going to be riding on private tracks then the Sticker issue is not a concern.
If you are going to be riding in forest areas then you need to have a spark arrestor and a green sticker.
In the desert, technically you need to have the appropriate sticker and a spark arrestor but you can usually get away without either. When the riding area is 50,000 acres and has hundreds of possible entry points it just isn't feasible for the rangers to inspect everyone. I have also seen them pretty much turn a blind eye when they happen by.
If you intend to ride in the forest areas, however, having a spark arrestor and the appropriate sticker is essential. In many of the forest areas you need to gain entry by stopping at the little ranger hut and paying your $5 entry fee. The ranger will step out and inspect your bikes, so if they are not legal you won't be bringing them in.
There are two stickers that can be issued to an off road vehicle: Red or Green. The green sticker is unrestricted, allows you to ride all year. The Red sticker only allows riding during a portion of the year, generally from late fall to early spring.
Any bike that is 2002 or older is eligible for the Green Sticker. From 2003 on the Green sticker is only issued to vehicles that meet the EPA requirements, which rule out all two strokes and even some four strokes.
When buying used you will find a lot of bikes available that have no sticker at all. It is possible to get it properly registered with DMV and thus get the appropriate sticker but it is a bit of a hassle. If you are only 14 then you probably haven't had to deal with DMV yet, so ask your dad about it. :nod: The last time I went through that process I had to make four trips to the DMV and two trips to the CHP, although it could have been done with two trips to DMV and one to CHP if I had known what the heck I was doing.
Long story short, factor the lack of a sticker into the value of the bike and price accordingly. I would have certainly preferred to pay a few hundred dollars more to buy a bike with proper registration then to take that much time off work to sit and wait at DMV.
As for the type of bike: A 125 two stroke would be a very reasonable bike for you. If it is new it won't be getting a green sticker so consider that if you intend to ride in the forest areas. The 125 will be a little bit aggressive for a beginner but if you survive the first day you will grow into it quickly and it is enough bike to last you a long time.
Rod