1. Expect rejection but don't be fazed by it. Use it to improve what you do.
2. Submit to honest competitions as often as you can. It is all useful experience in submitting work for outside criticism and in meeting deadlines. If you do well and win or are shortlisted, you can add that to your writing CV. And always check out the background of the competition so you know you are submitting work to a reputable one. It's not you, there ARE charlatans out there.
3. Be open to trying different forms of writing. Had I done this when younger, I would've discovered the joys of flash fiction that much sooner!
4. You can never have too much A4 printer paper or toner cartridges or pens. Stock up. Take advantage of special offers when possible.
5. Submit work to honourable online sites as well as for print anthologies etc. Your body of work will soon build up doing this and you cover both audiences - those who only read online, those who read "proper" books and most people go for both anyway.
6. Don't underestimate how long it will take you to be published. It always does take far longer than you dream of!
7. Before entering any contract, get it checked by the Society of Authors (UK) or other reputable equivalent body. You can save yourself a lot of heartache and money doing this.
8. Expect to be addicted to (a) notebooks, (b) nice pens, (c) going to good writing conferences, and (d) tea/coffee etc to keep you going as you write. Save up accordingly! Start now...
9. Read as much as you can, contemporary and classic, fiction and non-fiction. You may think you're already doing this but writing has made me read much more than I ever did before, sometimes for review purposes, sometimes not. You need to know what's out there now. It can help you find your own niche for one thing. You can then play to your strengths here which will give you a greater chance of success when approaching publishers.
10. Remember practically everybody struggles to find an agent, it isn't just you. Rejection is never personal either. It can be easy to forget these things. Keep going. There is a lot of truth in the saying the professional writer is the amateur who didn't give up.