Respect isn't especially difficult to earn, but it must be earned--that is to say, it can't be won, stumbled upon, or started with. When you first enter the workforce, most people (aside from those with a habit of casting immediate judgment) will view you neutrally, and the habits you exhibit will gradually move them toward one opinion or another; depending on your actions, you could set yourself up to be taken as a serious and professional member of the team, or a slacker unworthy of respect.
Obviously, you want the former. To be successful in almost any field demands that you are taken seriously, and engaging in these habits is the best way to get there:
1. Arrive early. There's no need to show up to a meeting an hour early, but give yourself a few extra minutes to get in and get settled--or at least protect yourself against the eventuality of getting sidetracked or detoured. This goes for more than just meetings, of course--it also goes for interviews, lunches, company functions, and daily work in general. Even if your company has a pretty lenient policy when it comes to punctuality, others in your office will notice if you occasionally show up late. Don't let a simple mistake like lateness compromise your reputation.
2. Be confident. While some people may be naturally confident, most of us have to work at it. The exhibition of confidence is going to be your greatest asset in your quest to be taken seriously because it shows that you take yourself seriously--and if you take yourself seriously, then others probably will as well. Practice confidence by being more assertive in daily situations, practicing good posture (see point four here, too), and refusing to be intimidated by the normal stresses of work. If you're not a naturally confident person, this can take some time to develop, but don't worry--practice will make perfect. For help with this, see 7 Ways to Appear Confident (When You're Really Not), and 7 Conversational Tricks to Appear More Confident.
3. Stay quiet until you have something good to say. If you slip up and say something foolish or inappropriate, it's easy for people around you to take you less seriously. Try to keep yourself silent in most situations unless you have something truly valuable to add. Even when you do have something valuable to add, think carefully to structure your phrases well and state your thoughts as concisely as possible. This will make virtually every word coming out of your mouth seem valuable, which in turn will make people take you more seriously.
4. Pay attention to your body language. Body language is important for more than just exhibiting confidence--it makes you seem more thoughtful, more put together, and more experienced in the professional world. There are dozens of things to watch for here, but some of the most important include sitting (or standing) straight, with your shoulders back, making eye contact when speaking to people, and shaking hands firmly (but not too tight). It's also important to keep your hands on the table and avoid fidgeting during meetings, and to allow your face to express your emotions during the course of conversation.
5. Prepare more than you think you need to. Whether it's for a job interview or for a simple morning team meeting, always prepare just a little more than you think you'll need. Do just a little bit more research. Practice your speech just a few more times. Come up with just a few more contingency plans. Chances are, you'll never tap into these preparatory materials to their fullest potential, but when you do get the chance to show off, you'll really impress people. Plus, if you make it a habit, you'll never show up to a meeting unprepared or underprepared--and that's going to make people take you far more seriously.
6. Read the news. This is a simple habit, but it's an important one. Try to keep up with major national and international news, as well as news within your own industry. You won't get a chance to bring up this information every day, but if the topic is ever mentioned by a boss, coworker, or client, you'll be glad you had a cursory knowledge of the matter beforehand. Eventually, you'll develop a reputation for being well-read, and people will naturally assume you're a diligent, focused, hard worker because of it.
7. Remain humble. I saved this habit for last, because it should be held in balance with all the other habits. Throughout your process of developing a better, more serious reputation, it's important to remain humble. Be confident--but never cross into arrogance territory. Show how much you've prepared--but never brag about it, or do so obnoxiously. Stay quiet--but don't avoid small talk and conversations with your coworkers. At times, this can be a hard balance to strike, but humility is an important trait in the workplace when it comes to establishing mutual respect.
Be sure to practice these habits on a regular basis. You might not earn everyone's respect overnight, but these habits will make an impression on people (even if they don't immediately realize it). Patience and consistency are your greatest tools when it comes to being taken seriously, so take your time and try not to slip up.