1. Invest in a good pair of shoes. The health and strength of your feet and legs are INVALUABLE. Therefore, you need to protect them with PROPER footwear. Go to a reputable running store, Like Inside Track or Appalachian Running Company. Bring an old pair of sneakers. They will look at the wear on the tread of your shoe and even watch you run/walk to recommend the type of shoe that is best for your body. Good running shoes are expensive. Expect to pay in the $100 range. But, if they last you several month, that's simply < $1 per day to have the proper footwear. That's cheap, to me!
Extra Tip: I know it's important to support small businesses, but if cost is a serious issue, there are some great sites online to get discounted running shoes. The shoes I wear continue to come out with "new" designs, (pretty much the only thing that is different is the color) so the "old" designs can be found online at a huge discount. I go to Holabird Sports. I have found the "old" design of my $114.00 running shoes on there for $59.95. I bought 4 pairs and they lasted me over a year! So bookmark the page and check back once in awhile for deals. Snag a few pairs when you can!
2. Walk/Run. If you have never run before, or haven't run in years, it's not a good idea to wake up one morning and try to run 3 miles. First of all, you could injure yourself and you'll probably be really sore for the next couple days! Not only that, but you probably won't be able to do it! (Or you will do it but you'll feel like complete shit after.) Instead, alternate walking and running. Example: Walk for 2 minutes, run for 1 minute. Keep this up for 20 minutes. Do whatever walk/run ratio you like, but as the days go by, you should slowly increase your walking time and decrease your running time. You'll be running the full 20 minutes (and even longer!) in no time!
3. Set short term and long term goals. If your ultimate goal is to run a half marathon or marathon, that is awesome! But set shorter goals in between. Your goal could be to run three times a week or it could be to run a 5k. Achieving short term goals will motivate you to continue to your long term goal.
4. Test out a few hydration packs before you buy one. It's always nice to have water available on a run, especially if it's hot or you're getting into long distances. There's lots of different ways to carry water, some of which are pretty expensive. So see if you can try out a friend's before you commit.
5. Embrace your new title as a "Runner"! Some new runners are hesitant to call themselves what they are- RUNNERS! Did you go for a run on purpose? Congratulations, you're a runner! That's all you need to do to become part of the club! There is no other criteria.
Do you have any tips for beginner runners?