You don't use nor can use an "App".
You must go through a broker/dealer- Visit the web site of a broker/dealer you know of, or one you have heard of or visit a local office of the firm. Schwab, Ameritrade, Fidelity, Wells Trade, or you can visit the branch of a major bank..
Regardless of how you do it, you'll need to compete a "New Account" form, deposit money into the account (the firm will tell you how much and how).
from The Street