Chances are that if you use a modern Mac system, especially a portable system, you will regularly be accessing at least one Wi-Fi network for connecting to the Web, email, and other on-line services.
If you use your system at more than one location, then you might regularly log into multiple Wi-Fi hotspots, be they private ones such as those at work, or public ones at cafes. Sometimes, at conventions or large workplaces, you might find yourself switching between different hotspots, depending on the signal strength available.
When you make these various connections, you might find various problems with Wi-Fi connectivity, where the system may prefer to use a hotspot with less signal strength than another one in the vicinity.
While there are various ways to troubleshoot Wi-Fi connectivity, if you regularly access Wi-Fi hotspots you can help your system pick the appropriate ones through several means.
First, most Wi-Fi connections broadcast their networks publicly, so you can see the network name when choosing a Wi-Fi network to connect to; however, this is not always the case. If you know you are in the vicinity of a Wi-Fi connection but cannot see it, try manually entering its name and password. To do this, go to the Network system preferences and choose the Wi-Fi connection in the list of services. Then in the Network Name drop-down menu, choose "Join Other Network." This will bring up a dialogue box in which you can enter a network name, and choose an appropriate authentication scheme for supplying a password.
The second option for managing networks is to remove any that you do not use, or that you do not wish to use. While you can have a massive list of previously-joined networks and the system will only choose from available ones, there are some cases where you might have multiple available networks and the system may join one with a slow connection or one which is otherwise less desirable.
To force the system to choose only the desired one, you can either remove unwanted networks, or prioritize them. To do this, again go to the Network pane of System Preferences and choose the Wi-Fi connection, but this time click the "Advanced" button.
In the Advanced options panel that drops down, select the Wi-Fi tab and you should see a list of preferred networks, which contains any network you have previously used. Now you can select one or more networks (hold the Shift or Command keys when clicking to choose more than one), and then click the minus button to delete them.
For prioritizing the networks, the system will select networks from those in this list, starting with the ones at the top. Therefore, by selecting a network and dragging it above another one in the list, you will be giving it priority. In this manner, you can locate the networks you connect to by name, and then select and drag them above others that might be connected to preferentially by the system.
When finished with these settings changes, click "OK" and close System Preferences to save the changes.