Here in our store, we often say that the difference between homemade and handmade is in the details. One of the biggest details to improve your knitting is to master grafting and seaming.
Kitchener stitch and mattress stitch are the 2 most common methods of seaming and grafting.
This video by Patons is a very good tutorial on grafting. It shows how to graft a set-in sleeve, which is terrific because it shows many of the stitch grafting combinations you will encounter in your knitting.
For a more in-depth look at grafting, check out this series of articles by Interweave. You may want to grab a cup of tea and be in a quiet room where you can concentrate before diving into these gems. These articles will not only help to improve your grafting, they will also help you understand the construction of knit garments and boost your ability to read your knitting.
Mattress stitch is perfect for vertical seams. Purl Soho put together this page with both written instructions and a video with excellent instructions for doing the mattress stitch.