How do you create a linear pattern? Can I use any feature?

There are two places where you can make use of the Linear Pattern: while sketching and as a feature.
When you know you are going to make an array of one particular shape, it may pay off to sketch that shape and make a linear pattern of it by selecting the sketch and a line or linear shape that has already been made, which will be the direction in which the sketch will be multiplied.

You can pattern features as well, except for fillets and chamfers, which aren't really features but edge editing tools. Use a sketch line or a straight edge from geometry that you have already made, to define the direction in which your feature will be multiplied.

Note: to avoid problems, you may choose to uincheck the 'merge result' option and then apply 'Combine'-feature after executing the linear pattern to merge the result.

http://www.solidworkstutorials.com/2009/01/how-to-use-linear-pattern/

You can pattern fillets and chamfers if they are part of the feature you are patterning, just select them in addition to the feature you select to pattern

You can pattern within a sketch but be aware that the pattern is not constrained, the easiest way to constrain it is to hit the "fully define sketch" tool

VIDEO 3. MINUTES

Since it is one of the easiest things to do in most CAD programs, I suggest you begin to use a playlist from Youtube like this one.
https://www.youtube.com/watch?v=ONcM1acrJYQ&list=PLSzVwzoXi_Uv7CQPJoxSi5w74Oz4LhFgl&index=17
That link takes you to the linear lesson, but you can access the full playlist by clicking on the channel name. Start at the beginning and go through every one. I have also used other channels: Onnova Systems, Infinite Skills, Go Engineer etc.