I have several writing friends that I go to depending on the type of piece that I am working on. From years of knowing one another and writing together, I have learned that their criticisms and tips are things that have fostered my writing along to a publishable state. Writing friends should be people that you trust, but who are not afraid to tell you when something doesn't feel right in your story. If you want encouragement instead of critique, then grandparents and relatives are a great resource.
My writing friends and I usually like to meet one-on-one with each other over coffee or tea and talk out impressions of my story and things that can make it better. We start with the big picture stuff, which is making sure that the plot and characters make sense and resonate with the reader. Later on, when I'm getting closer to sending it out into the world, it's nice to have them check for errors or awkward sentences. Writing friends are great for the brainstorming stages too or when that writer's block hits. I've spent countless hours at coffee shops with my notebook out scribbling away as my friends give me ideas and I bounce mine off of them. My first novel was actually planned in this way during a fantastic three hour brainstorming stint at one of my favorite burger places.
One thing to keep a healthy writing relationship going is to realize that it is not one sided. You can send your friends your stories as much as you want, but eventually they are going to get tired of always giving you feedback when they never get any in return. Offer to return the favor and then, here's the kicker, actually do it! Read their work closely and give detailed feedback, the kind that you want them to give you.
Finding that writing buddy can be easier for some folks than others. I was lucky in that I connected with a lot of people in my college English program and we have stayed in contact over the years. I know that others find their writing friends through Facebook groups or community writing clubs. You have the Internet at your fingertips, don't be afraid to reach out and see if you can find some people to exchange ideas and stories with. Most likely, they will be happy that you asked.