For anyone who is starting to get into bondage, it can be a bit tricky to start learning the bases. Something like knots, for example, are essential to the lifestyle…Alas, is not something they teach at college (other useful things that they don’t teach? Taxes, anyone? Wink*) The good thing is with a bit of theory and practice you can make some knots that will make any sailor jealous.
Before trying out knots, it’s important for you to know what exactly will you be trying. You will need to choose your rope according to the activity, this is not a one size fits all (how many things like that are, really?) If you want to bind limbs or make body harness, 3/8 or 5/16 rope will work nice. On the other hand, suspension bondage needs a really strong rope, obviously. If you are into CBT, keep in mind a thinner rope would be preferred.
As far as materials go, nylon and polypropylene are the most popular, but silk ropes are also very easy to find. Marine rope is commonly use for suspension because is really strong and durable. Cotton holds knots well 
and does not stretch, but breaks down easy so keep that in mind depending on your planned activity. Jute and hemp are not of common use, unless they are being used as more of a punishment because they can be hard on the skin (they are also used as floggers, if you are interested wink*)
Ok, so now let’s read about knots. The first one you should know is the square or reef knot. Is simple, it binds two ropes or creates a restraint around a wrap. It does not apply much pressure to the skin because it’s flat; its my favorite knot if you are a beginner (we were all once bondage beginners, wink*)
If what you are looking for is tying someone to the bed, or some other object (like a chair) the half hitch or the alternative clove hitch are the way to go. They are easy to untie, which comes in handy if you are in a hurry after all that foreplay, wink*. But I should warn you, never use them to tie up a limb or for suspension because they can cut off circulation.
The third knot I will tell you about is the French bowline. This one creates a loop that does not slip; it will not loosen or tighten under pressure. It’s usually preferred for biding ankles and wrists together. The good thing about it is that you can use as many loops as you like, in order to make the restraints as wider as the area you want to bind. Oh! And it’s also safer and very comfortable (speaking from experience, wink*)
As a last few words of advice and warning is my duty to remind you to be careful. Communication during bondage is key, and it goes for the one being tied up as for the person who is doing the binding. Be careful about letting the ropes loose enough to allow circulation; remember, ropes are meant to restrict, never cut off circulation. Never leave a person you have tied up alone in a room and always have a pair of scissors or a sharp object nearby, just in case of emergency. Always check for any color changes in a person’s limbs and any variations, especially tingling or lack of sensations in their bodies). If this occurs, most of the time immediately (and calmly!) removing the rope will be enough. But in extreme cases, those pair of scissors come in handy. And lastly, just remember, practice makes perfect.
