In order of effectiveness:
Text Message: minus 100
email: Nil
letter: 1
Phone call: 5
Face to face (if you can get then to agree to meet you by phoning in advance): 100
Prepare a 20 second pitch about yourself which will make them think that you are what they need. Practice saying this to anyone who will listen. Make sure you understand what a potential employer is looking for.
Learn it (don't write it as a script) and have a written note of all the points that you want to make when you get to talk to someone, and questions you will be asked, including what you have done in the past and want to do in future, in a notebook. Carry this notebook everywhere with you. One day your uncle (or someone) may say to you in a pub or outside the church
or working mens club
"Meet my friend Bill, he has a small firm. Didn't you say you were looking for experience?"
Tell all your relations and older friends (i.e. people who have been working for a living for some years) what you are looking for and ask if they can suggest anything. This is better than asking if they can give you a job. Write down in your notebook who you spoke to, when, and what they said. Give them your phone no (written down with your name and "Plumbing Work") and try to get them to put it in their wallet if there is a chance they may get an idea. You will be surprised at the contacts people have. Even your mum will have a friend whose husband is a plumber.
If you know anyone who is any building trade he will know someone. Involve your friend's parents. Tell them what you've done and what you're looking for. It will be good practice for you to speak to older people sensibly and with respect. people like to help, but only if they think you are a good candidate. otherwise they won't want to spoil their reputation.
If anyone gives you a lead, thank them, write it down, and phone within 2 days. If they find out you didn't, they won't bother trying to help again. When you phone a lead, always introduce yourself by saying "Bill Bloggins suggested I call you, he's my uncle and I believe you know him through the pigeon-fanciers' club (or whatever) and they will be more inclined to listen to you. Bill will be asking them, next time they speak, if you called and what he thought of you. The lead will also be asking Bill what he thinks of your character (skills can be learned, character can't).
Have several copies of your CV in clean envelopes and carry them with you. Don't give them to your contacts, only to people you've spoken to and have shown an interest as potential employers. The envelope will help keep it clean and smart.
If you look scruffy, lazy, dim, dishonest, criminal, doped, drunk, hungover or unpunctual, you haven't got a hope.
Don't dress like a student. Look like what you want to become. It might involve a haircut. Remove visible body-jewelry, cover tattoos.
Your college should have been helping you find placements. Ask how they can help or advise you.
p.s. one of the reasons people will not seem interested will be that they might have one vacancy per year, and they get two thousand people asking. You have to make then think you are better than most of the others. But be aware that most of the others will have got one or more of the "no hope" characteristics, so it will be easy for you to beat them if you put your mind to it.