I have a piece of pine that's 1/2" x 3/4". I want 1/2" x 1/2" and I can't seem to find that at the big box stores. For tools I have a jigsaw, circular saw but that's it. I don't really have money to invest in a bandsaw. What else can I use to make this smaller? Hand planer? I'm using this for an extension jamb on a window
Your circular saw will do the job, but you should make a jig to guide the saw instead of trying to follow a pencil line. You will need a couple of pieces of straight lumber a little longer than the cut you want to make. Many books and articles say to use scrap wood but I sometimes use good lumber destined for use later.
The technique is to cobble together an assembly with an inside corner that the saw's foot plate can be pressed against, with the piece to be cut positioned in the path of the blade. Sometimes you can clamp the assembly together but I find that the clamps get in the way so I usually use small nails.
The shape of the jig will depend on what material you have on hand to make it. Here is a suggestion:
Please forgive the crude cross section but it should convey the main idea.
Lumber yards and big box stores that sell lumber usually have a trimming saw that they use to cut customers purchases to size. Of course they probably won't cut a piece that you bring in, but if they don't have a 1/2" x 1/2" in stock, you can buy a 1/2" by something else and they will cut it down for you.
Doing this with power tools safely would probably require building a jig, as mentioned. A hand plane would take quite a while to remove that much material.
A simple miter box with a hand saw could be an easy way to do it. Many of them have mechanisms to clamp a piece of wood in while you cut.
Or if it doesn't need to be perfectly square, you could just draw a line, clamp it to a table, and cut with a hand saw - a nice pull saw would actually let you cut it pretty clean.