Some agencies require experience, others don't. Be wary of any advice they may give if you are new, because it is not always good. Many agencies require pics before they will interview, but if you feel strongly otherwise, stand your ground. Don't send pictures or an application without talking to the agency first, if only to confirm that they exist. Don't work with an agency that doesn't want to meet you.
Education is a double edged sword. On one hand, many clients want reassurance that you are not going to end every sentence with "innit." Conversational skill are an excellent way to ensure a long booking, but conversation should be on the client's terms. However, giving away too much info on this can reveal your identity. It can also make you look unapproachable to some clients; many of my clients are not necessarily what one might call educated, but are fantastic and lucrative. Beware of any client who compliments you on your intellect, or any other personal qualities e.g. being nice, caring, thoughtful etc. Obviously smile and say thank you, as the compliment may be genuine. However, all of my clients who have wanted things they shouldn't (bareback, my real name, a relationship) have made these sorts of comments.
If you work full time for an agency, you should expect to earn a living. If the agency cannot earn you a living, they owe it to you to be honest. Agencies can give you a good idea of when to expect calls; it varies by area. You are self-employed and can work whenever you want and with as many agencies as you want, and anyone who tells you otherwise is not someone you want to work with. This is, however, a business trasaction and you should work when you say you are working.