staion
Currently I am using Amazon AWS EC2 on the 'free tier'. I thought I was only going to get a 1 month of free service, but its been well over 4 months now and I still don't have a bill. Granted there hasn't been much traffic and I don't host files or pictures on the server which keeps the bandwidth down.
Its still very convenient albeit I am not using a domain name, but for development, the ability to stand up a Ubuntu or Windows server for quick SSH access works for me.
To get a user base you need to host content that appeals to a specific topic. You need 0-day information, crafts, opinions, hobbies, recipes, anything that no-one else has. Reddit and major news outlets will always be your competition. So maybe spend time there and kick up discussion then lead that discussion to your forum.
I think there is a lot of benefit if you can get people you know or who are in your RL community to join. Do you live in a small community? Maybe create a village forum. Do you visit a coffee shop alot? Maybe try to get people in the coffee shop to share on the forum.
Take a photography city college class and share your photos with the class on your forum. Other students join and post their photos, and the cycle continues. You just need to find what your unique content is going to be. Typically is something you are passionate about. If you share it, they will come. ?