Getting started with ballooning usually begins with a familiarity with the basic components of a high-altitude balloon and educating yourself about basic costs associated with sending up a balloon. Let's begin with a few of the things you'll need to send up a balloon.
The most obvious (and sometimes the most overlooked) component of a balloon is the balloon itself. When it comes to choosing a balloon, you need to establish a target altitude and choose a balloon that is best suited to traveling to that altitude. Balloons are rated in terms of weight, and they are engineered to burst when they reach a certain diameter. Hwoyee, a popular brand of balloons, has published a helpful chart that will help you to select a balloon to best fit your purposes. This chart can be found here. We usually use a 1600g balloon for scientific missions.
When it comes to the choice of gas to fill the balloon with, the amateur balloonist is presented with a choice between hydrogen and helium. Hydrogen is, theoretically, the best gas for usage in balloons, as it provides twice the lift at (roughly) one-half of the cost as helium. However, many balloonists opt to use helium due to the explosive and flammable nature of hydrogen. Ultimately, the choice is up to you. It all comes down to a matter of preference and risk analysis. Also, take into consideration the cost of the volume of the gas based on the size of the balloon.