It's very difficult to just 'knit in patterns' on your own. I suppose more experienced knitters can make up a pattern, incorporating different stitch patterns. There are, however, lots of patterns already written out there that have all kinds of designs built in. One place to start is www.knittingpatterncentral.com
for free patterns from all over the web.
There are also hundreds of pattern books and booklets out there, too.
If you find a sweater pattern you'd like to add something to, there are books that have stitch design patterns you can add, also.