It sounds like you are starting with hand tools. Lots of help available in the Hand Tools forum.
Many people do not attempt to cut a perfect 45 deg mitre. They cut and then tweak on a shooting board with a hand plane.
Thanks for the helpful link Dave! This is really informative. They don't tell you about this stuff when you're at Home Depot buying a $7 plastic mitre box/hand saw combo pack.
I have had to use this technique now and again, even when I have cut on the table saw. In this case, it was not my angle being off, but my length not being exactly consistent.
I was in San Francisco with my girlfriend and took a photo at this booth.
It was great because it was using photo paper and chemicals to develop, fix and stop for a print out. They just don't make them like that anymore. The photo that printed out was 1.5"x8". Because of the sentimental value, I wanted to frame it, but neither Aaron Brothers nor Michael's could do anything that thin. They told me there was a minimum width of 7". So I just decided to build the frame myself. How hard could it be right?
After getting a few cheap handtools, I got frustrated with how inaccurate my cuts were. And decided to upgrade on a few woodworking tools. After borrowing my brother's table saw, buying a router table, and building a table saw sled, I managed to finally put this bad boy together.
It isn't perfect, but it was the start of this desire to build stuff out of wood and start learning the tricks of the trade to build things myself. I think the next project will be a bedside table.
Again, thanks for all the useful help along the way.