Speaking at conferences is a great way to share ideas and meet people - but actually getting the opportunity to do is a little more tricky and usually involves proposing a talk. In the last year I've attended IPC in Germany and PHP London, spoken at DPC in Amsterdam, submitted talks to and attended ZendCon, and helped select the sessions for phpnw - so I've seen it from all angles.
The first thing to say about submitting talks, is that there are no pre-requisites. You don't need to be published, well-known, or have letters after your name (in the PHP community, the latter is probably more hindrance than help). If you want to go to a conference, and there is a topic you'd like to share some thoughts on, then write them down and submit! A lot of conferences have a Call for Papers - usually this will be an online form where you put in your personal details and the details of the talk you'd like to give. If it sounds simple, that's because it really is ...
Proposing your talk
It can be tricky to know what to write in the boxes and how to sell your talk to the conference organisers. The call for papers should give information about the themes of the conference, the expected audience, and the kind of content they are looking for - so pay attention to this. Usually you'll be expected to submit an "abstract", this is a description of your talk that will be put on the schedule if you are accepted. A good way to get started with these is to read the abstracts from current conferences - these are the ones that got through the selection process and will give you a good idea of what you should say here. Its usual to also be asked to supply a biography, either when you submit your talk or when the talk gets announced as part of the conference schedule.
If there is room for additional information, then give it - and give the organisers as many opportunities as possible to feel like you would be a positive and safe addition to their event. I've seen a few variations on these but for the phpnw call for papers, we added a box which we didn't publish the contents of and where speakers could tell us why we should have them and/or their talk. This was illuminating, responses varied from "because this topic is so cool!" to "not sure really, thought it might be interesting though" and the unforgettable "meow" (that last one was from an entry that didn't get accepted - it was hard to tell if the speaker was taking the whole thing seriously or not).
My advice is to start planning your submission in plenty of time - take a look at the information that you will need to supply and make sure you have it all (and do write in the optional boxes). Its also a really good idea to bounce your idea off some other people, who can help proofread and point out any obvious problems with your submission - for example the time I tried to submit a talk to a PHP conference without the word "PHP" anywhere in my proposal ...
Getting your Talk Accepted
I have yet to successfully submit a talk via a Call for Papers and be accepted to speak at a conference - so I have no idea how to get talks accepted. If anyone else can add advice on this topic, that would be great :)