Amateur groups are worthwhile for lots of reasons. If you are good, you can get loads of shows and events under your belt very quickly. Consider joining two or three. Apart from the actual performing side of things, they're also very good at levelling people. Some of these groups are very tricky to work with because they have rules, they have very set ways of doing things and although your talent gets you up the ladder quite quickly, most do very strict auditions which will help you greatly when it come to paying auditions. You will always find somebody who you feel isn't as good as you getting the role you wanted and you learn to fight hard - all great experience. Some amateur groups are really social groups, plenty of fun times and maybe the actual productions are a little er, well - rubbish - but that doesn't matter, as it the process experience. Others are professional companies in quality and attitude, but you don't get paid, yet they spend thousands on each show. These tend to be quite cliquey - but again, not always a bad thing.
Agents are a very different matter. Pretty well your school experience counts for little, so what do you have to enable an agent to market you? I'm guessing, but maybe a few bits and pieces on paper, and maybe a photo? Is the photo a proper industry typical headshot, and maybe other similar full lengths? The kind of thing they could send out? If they don't have anything - then an agent won't represent you. Remember agents make money by getting you work, so they need to know what you can do in a proper form - and that means experience. Younger people who have had jobs in the industry usually make up packs of information showing what they've worked on and most importantly - who the producers/companies are. There are bad agents, of course, but if they don't get you work it might be that they're pushing other people, not you, which doesn't mean they're bad - they just stand more chance of earning money by somebody they think has a better chance. So my advice is that unless you have some kind of feature that makes you 'bankable' then at this stage, an agent isn't needed. Don't pay agents to be on their books, it shouldn't work that way. They make money from your work - if they don't get you any, they lose!
Now the bad news.
nobody actually belives i could become an actress
Think long and hard about this, because it's important. Two things could be happening. You could be a good actress, and they're wrong, or you may be a terrible actress and they're right (and just trying to get you to be realistic). You say you've had quite a lot of experience at school and 'stuff'. What exactly have you done. If you want you can list what you've done and we'll tell you the status and importance of these, career wise.
Hopefully the 'stuff' will be the local amateur shows - where you've perhaps been in the chorus, or maybe you've been the one in a dance show who had a speaking role in a little segment - that kind of thing? You want to be an actress, badly - so this means you must have already done it, and learned your lines and performed in front of the public - because if not, how do you know it's the job for you? A bit like me telling people I want to be a dancer. Something anyone who knows me would have said forget about - because I have no talent whatsoever in this area. Wanting to be one would have been unachievable for me - in fact, for me - stupid!
So you need to think about what you have done, and what the quality and standard was. Then consider if your need to be an actress is based on your utter brilliance and natural skills, or just something you feel would be nice. Wanting this is fine - expecting it can set you up for a big fall.When I had people come for auditions at college, aged 16, who told me they knew they were going to be so good at musical theatre, I'd ask them what shows they'd been in. Some slapped down a portfolio full of them performing from almost being a baby. Others said they were in the chorus for the school performance of Grease, but didn't have any other info. If I had only a few places left - guess which one didn't get an offer?