The funds that you have in your Broker account, Zerodha, will ideally reflect here in smallcase. So, you'd need to have sufficient amount in your Zerodha funds to place orders in smallcase.
For all the buy orders, the funds will be debited from your Zerodha funds and in case of sell orders, the amount you realized will be credited back to your Zerodha funds. But, you'd be able to withdraw this realized amount only after T+2 days.
You can view the available funds by clicking on your Account avatar on the top right corner > Funds.
Clicking on the Add option should take you to the payment gateway page where you can transfer funds to your Zerodha account. In case of insufficient funds, at the time of placing orders, you will get the option to add funds from the same page.
Here are some links that can help you with the fund transfers,