How to choose bicycles for low level riders:If you go to the larger sports shops or bike stores they will have complete bicycles for sale, usually an alloy frame with Shimano Tiagra or Campagnolo Mirage or a mixture of not much expensive components, These bikes will be built to a price, will ride quite well, wont be very light and is a good point to start. To get something better, you might start with a good alloy frame and if you can afford it, carbon forks, then with what money you have left chose your group-set of either Campagnolo or Shimano, and then you can pick your handlebars, saddle, wheel rims and tires, this is very funny, however if you are working to a budget, it can be tricky.
Local Bike Shops (LBSs) generally all sell reputable brands. Most brands use similar components, so the major difference is often the frame. Look for a brand that has a good warranty, preferably a lifetime warranty, on the frame. Mountain bikes have become a very low-margin business, so many manufacturers are running in the red or may already be out of business; its preferable if yours isn't one of these. Still, unless you end up replacing the frame, it won't matter much over the long run what the brand is.
Also, there is a growing trend of extremely low-end manufacturers buying out reputable but cash-strapped brands purely for the name. They can then sell junk bikes with a good brand name on it. These bikes are usually sold at discount or sporting-goods stores, not bike shops.
start with the seatpost itself, most of them have the size stamped on them below the minimum insertion hash marks. It is almost always measured in metric, millimeters usually, like: 25.4mm (which is 1 inch).
If the size is not stamped on the seatpost (it was most likely cut off or scratched beyond recognition) then you can measure the diameter in inches, multiply by 2.54 to get the metric measurement in centimeters, multiply that by .10 to get millimeters and then call your local bike shop for a replacement.
Dyno bikes are made by GT, it was there "budget" brand of BMX bikes, so you can google GT bikes and find a local dealer or you may be able to order direct from GT. From the year of your model, most likely the bike is steel (cro-moly or hi-ten) and these bikes normally have a smaller seatpost size, like the above mentioned 25.4mm. Aluminum frames usually have larger diameter tubes with thinner wall thickness and will require a larger diameter seatpost.