I know have about 40 dollars left. Let's say I feel really good about risk management and want to buy a stock that's about 45 dollars. Can I do that?
You absolutely should not do that. What you're talking about is "investing on margin." Generally, the way this would work is your broker would lend you half the money. Considering a 50% margin requirement, you pay $22.50 and your broker lends you $22.50, you pay interest on this $22.50 and if the stock falls below your margin maintenance level ($22.50 in this case) you will either have to contribute more to the account (known as a margin call) or risk the sale of some of your account assets in order put your account back in to alignment with the 50% cash requirement.
Put the remaining $40 in to TD's money market or some broad index fund with a low expense ratio that will take a $40 initial investment. Contribute to your account each month and when you have the cash sufficient to support another position, take that position in cash.
Do not play with investing on margin until you are comfortable with the movements of the market and have a sufficient cash position in the account to absorb the daily ebbs and flows of your investments.
As an aside, I am a big advocate of playing with real money not play money accounts. But you need to get used to your 3 shares of Tesla moving around every day before you start to really ratchet up the risk and borrow someone else's money to invest with; especially when the lender has the authority to unilaterally sell your positions as is the case with margin investing.