Get someone to hire you to manage a campaign. Honestly though, you're really going to struggle to do that. There's a lot of managers and wannabe managers now, and not many new campaigns starting (unless you count those scam ICO bounties).
And that's the truth. I myself, someone who's barely starting out falls in the wannabe category and Darkstar_ falls in the real deal category. Does not necessarily mean that either of us are any less or more trustworthy than the other, its all about the number of campaigns we have successfully managed in the past.
Even if you get 1 campaign under your belt, getting the next one is considerably easier. However, you have to be transparent about everything and although its not a hard fast rule, but it helps if you are a long time forum member (2+ years) or a high-rank member (Hero+). But if you can get in touch with people behind a new bright ICO and if they trust you enough, you could get to manage their campaign. Its all about making the right connections, convincing them of your trustworthiness
and knowing what you are doing.
PS: Everyone believes that they are trustworthy, it is down to your convincing skills on whether you can convince someone else of your honesty/trustworthiness. Start small work your way up. Good luck.