Determine if You are Eligible
So long as you are 18+ and not currently considered bankrupt, you are eligible to invest.
Open a Central Depository Account
Most brokerage firms will help you set up a central depository account, or CDP. This account provides clearing, settlement, and depository facilities for you to work with. You can create the account yourself or get your chosen firm to help you with it.
Choose the Right Broker
The typical aim when choosing an online brokerage is to find one that has a low fee. However, you also need to consider things such as reputation, ease of access to the trading platform, investment options, etc.