1. Learn to use a Limit order. This will also improve your understanding of Market orders, as you'll see the difference.
2. What stock are you trading? Try practicing with small orders on a low-priced stock with a tight spread. The spread between the bid/ask price can be quite large for some low volume stocks. Look for a 1-penny difference between the bid/ask prices.
3. Not advice, but Ford (NYSE:F) might be a good choice to practice on. At the moment they have a bid/ask spread of 1 penny: 14.95/14.96. Generally pick one in the S&P 500 they tend to have tighter spreads.
References:
1. Wikipedia: S&P 500 companies:
https://en.wikipedia.org/wiki/List_of_S&P_500_companies
2. Investopedia: Limit Order Definition:
https://www.investopedia.com/terms/l/limitorder.asp