Calm down! easier said than done. Remember you know more than they do but do not put on a suit that does not fit. If you get questions you cannot answer, do not get nervous, simply say: it is a good question and you will check up the answer. Don't start putting out excuses that you are inexperienced, haven't done this before, or whatever. Let what you do speak for itself, I am sure it is more than fine.
Now that the emergency is over, what can you do? Be prepared. Check out the material ahead of time and if you are in a lab or have to help out in exercises, run through them yourself to figure out where problems may arise. This takes a little time but will help you run things smoothly. In the case of a lecture situation, you also need to be prepared, read up on the material, make a good series of slides. Show what you plan to do to someone more experienced to perhaps get feedback and pointers on improvements. A typical rookie mistake is to overwork things, put too much stuff into a lecture and set the level too high. Assessing the right level is something you learn but if you have a chance to look at others' lectures or perhaps go back to what you experienced, you may be able to find your level quicker.
If yo want a quick check on what you have done you can do a quick feedback at the end. Give each person a small piece of paper (e.g. index card) and have them write one or a couple of things they found good on one side and something they think could be improved on the other. Do not use the word "bad" on that side, you want to know what you did well and what might improve. Collect the feedback and check it immediately. It may help you get a sense of how you came across. You can do this on a regular basis if time permits.
In the end, it is tough to get started, we have all been there with different amounts of self-confidence, experience etc. Expect some things to go less well, it is bound to happen, we all have off days. If something is less well, use it to improve. Use your colleagues as support if you find it rough, everyone has some experience of that sort.
I wish you luck. It can be really fun to teach. Look for pointers on teaching on the web and check out journals for higher ed. teaching to get ideas. There is no reason to approach teaching less seriously than what one would do, say research. The butterflies you will inevitably feel when you start will disappear and your confidence will grow with experience. Just keep calm and on top of matters.