Hello there! It's been a while but, BlenderNewbies lives!
In this topology tip, I want to cover a very easy, but not so obvious, way to get rid of the collection of triangles found on capped cylinders and when you close a circle of vertices. Here's an example of what I mean:
As you may know, it's best to use quads (four-sided polygons) as much as possible in your modeling. In this short study, we'll be converting the triangles shown into quads. There are a couple of important points to note that make this a little tricky, but we'll get to that shortly.
Let's BeginWith your circle of triangles in place, press the A-key until all of the vertices are selected. Then, press Ctrl-N to recalculate the Normals so they all point outward. (Sometimes vertex/face normals can become flipped during the course of modeling and this can cause a number of issues later on so, it's always a good practice to just make sure they all are pointing outward.)
Press Ctrl-Tab to switch to the Face selection mode, making it a little bit easier to select the faces.
Now, as a test, let's see what happens if we select all of the inner triangles and try to convert them all to quads (Alt-J).
You can see that we still have some unconverted triangles. Not good! I'm not sure why all of the selected triangles aren't converted to quads but, let's focus on finding an immediate solution.
Press Ctrl-Z to undo until you get back to having all triangles again. Press the A-key until all faces are deselected. Now, select just two adjacent faces as shown.
Continue selecting pairs of adjacent faces while leaving a pair of unselected faces in between. You should end up with the following selected faces.
Now let's go back to the Mesh menu to try converting the triangles to quads again.
The selected triangles have been successfully converted to quads.
Now, simply select the remaining triangles and convert those to quads.
Here we have the finished version consisting of all quads!
Here's a quick video of the entire process: