Where to locate the code snippet?Place the code snippet at the beginning of the body section (after the opening <body> tag).
You can read more about adding the code snippet here.
Which actions/properties to track?You should track meaningful interactions of the user with your website. Any action that you would like to get insights about (and improve conversion for) needs to be set as goal. Actions that may contribute (positively or negatively) to the goal should be also tracked (as non-goals).
Following are some best-practices but they may not be good for - eventually you know best what is relevant to your business.
Registered user versus visitor
It is recommended to track whether user is a registered user or visitor. You can see a specific example here.
Days since signup
Finding a statistical connection between the number of days since signup and your goals ('buy', 'share', 'comment') can be beneficial since you can focus marketing efforts on specific users according to their contribution to your goals. You can see a specific example here.
For e-commerce websites it is recommended to track when user completes a transaction and the total amount spent (example here). Additional information about the customer that you have from previous visits (if she previously signed up) can help find connections between the user characteristics and the amount spent. For example it can find that users spend the highest amount on their 3rd visit and afterwards the total amount decreases. This can help you focus on customers that bought twice in the past, as they are more likely to spend most on their next visit.
If you have social information such as age or gender, Zoom Analytics engine may find insights on that as well. You know your business best so you should track anything you think might have an effect on conversion. If later you see that the property doesn't provide meaningful information you can remove it. Any meaningful historical information about the user that you know after she signs in, can be added as property.