My advice is similar to the two previous posts. I found it easier to mount and ride off using a solid object (rail or fence) rather than holding on to someone.
Use a rail or fence to get comfortable with the uni. I used a homemade rail:
When you can ride along the rail with very little support, it is time to ride out in the open - or at least to try… 
Do this repeatedly and you will gradually get farther and farther. At this stage I sometimes got tangled up in the pedals and especially my right calf got quite bruised - but most of the times I just stepped off the uni when I couldn’t keep the balance any more.
Just stick to it - you will feel very happy about yourself the first time you ride 10 revolutions or more! 