Webinars can be divided up into two types: those where you can talk (and perhaps share video via your webcam) and those where you can't.
Webinars where you can talk
In smaller group webinars, you may be given the option to enable your microphone and webcam and therefore be able to participate verbally. Some platforms have a hands up button that you can click on to show that you would like to contribute and whoever is hosting the session can give you the 'floor'. This is often needed in medium-sized groups to prevent people talking over each other.
Even in these webinars you can also participate by typing into chat windows. These are the place to ask questions, share short thoughts and often links to web pages and documents. They are not automatically opened by some platforms - so look for the word 'chat' or an icon of a speech bubble or something similar.
Webinars where you cannot talk
For larger webinars, it becomes unwieldy if everyone has the ability to participate verbally and therefore this option may not be available. If this is the case, then your main way of contributing is via a chat window. As mentioned above, some platforms do not automatically display their chat windows so look out for the word 'chat' or an icon of a speech bubble or something similar.
The presenter may ask you for answers to particular questions which you need to type into the chat window, or you may want to use it to ask questions of the presenter.
Another feature of larger webinars where you cannot talk may be polls. If the presenter wants to gauge opinion on something they could ask you to vote in a poll. These are usually anonymous.