A transaction is a series of actions/requests that represent a meaningful business or user value. e.g. account creation, newsletter subscription, order processing etc. Transaction goal is the number of transactions handled by your site per minute. So you need to estimate the number of transactions per any period of time, calculate the number of minutes in this time period, and then divide the number of transactions by the number of minutes.
Let’s assume, your website is expected to have 600 logins per 1-hour period. It is equal to 600 login transactions per 1 hour, i.e. per 60 minutes.
To calculate Transaction goal for the web site you need to divide 600 transactions by 60 minutes. The resulting value of 10 is your Transaction goal.