After getting your Demat and Trading Accounts you will not be having any Balance in your Trading Account. In order to buy shares you should compulsorily have money in your Trading Account. If you don't know how to transfer funds to your Trading Account Then read this post How to deposit (or) Transfer funds (or) Money to your Trading Account.
After depositing money in to your Trading Account Log-In to your Brokers Website or Online Share Trading Software which has been given to you to buy shares online by using your Client Id (User ID) and Password. After Logging in to your Demat Account first check the available balance by clicking on Account Summery. If you have balance then proceed to buy shares. To buy shares first Launch the Market Watch Window and Insert the Script or Company Name you want to buy. you can load up to 100 Scripts in a single Market Watch Window. After Loading the Scripts watch the script carefully. Price of The Scrip changes in every second. When you decided to buy right click on the scrip and click on buy. Then One Special window will be appeared to buy shares. Enter the Number of shares you want to buy and then enter the price at which you want to buy. you may opt to buy at market price or lesser or greater than the market price. After entering the price and no of shares press enter key, then one confirmation window will be opens displaying No of shares you wanted to buy and the price. Verify and confirm it. Then order will be taken to buy. When the price at which you placed order comes it will buy the shares automatically. You can check those shares in your Demat Account later by clicking on Portfolio.