Pick yourself any basic hosting package for a forum start up. Godaddy, 1and1, fasthosts, 123reg, namecheap etc etc. We have thousands of XenForo installs on these basic services who have no issues. Just be aware that in the future you may need to move to something more specialist / performance based. But to get going most cheap shared hosting packages will be fine.
Just compare the hosting requirements of Xenforo with your hosting. Chat with them or start a ticket. I have friends who recommend to use popular, reliable and big hosting. Just to make sure that they give good support and won't melt away one day.