Starting from Scratch
After opening Gmail, in the "Search all conversations" field, click the icon that looks like a stack of three horizontal sliders.
Fill in the info you want to filter for (e.g. "From: firstname.lastname@example.org"), resist the urge to click the big blue button that says "Search," and instead click "Create filter."
You have quite a few options for what to do with the messages that match your filter, but my favorite is to apply a label.
Check the box next to "Apply the label...", click the dropdown menu and either choose an existing label from the list or create a new one. Check the box next to "Also apply filter to matching conversations." and click "Create filter."
Once you've created the label it will appear in the sidebar under Categories. This is where you can, by clicking the three vertically arranged dots that appear when you hover over the label name, change the label's color, edit its title, and/or change its hierarchical relationship to other labels.
More About Creating Labels
You can create any label at all, name it whatever you like, and give it color, too.
You can also create label hierarchies. For instance, if you have a complex project with many different parts, you could create one label that has the project name, and then nest the different sub-labels beneath, like so...
You might already have the concept of "categories" in mind, and you might think labels are the same, but they're not. A category is something you put other things into. A label is something you attach to other things. A limitation of categories is that, for things that might belong to more than one, it's often hard or impossible to put a thing into more than one category. Labels, on the other hand, can be attached any thing at all, and any thing can have as many labels as needed. This makes filtering so much easier.
Other Actions, too many emails
You can automatically delete emails that annoy you, but while that's oh, so satisfying, it's seldom practical. Another option for dealing with too many emails from a particular source is to automatically archive them. That's the "Skip the Inbox" option at the top of the list. Probably, you'll want to combine this option with "Apply the label" for easier searching later on. This is a nice way to avoid frequent interruptions, and instead review the messages on your schedule.
Starting with an Email
For some filtering needs, you might find it's a bit easier to start with an email. Open an email that you'd like to filter, maybe one from your boss, click the three vertically aligned dots ("more"), and choose "Filter messages like these." That'll save you a step.
Once you've created a filter and applied an action to it, all future matching emails will have that action applied to it (a label in this case).
And/or you can also apply labels to an email manually.
Edit Existing Filters
You can edit (or delete) your filters by going to Settings --> Filters and Blocked Addresses
That should be enough to get you started, but here are a few links with even more information...