As per semantic versioning, you should start off at 0.1.0
and increment the MAJOR version for any updates that are not backward compatible, the MINOR version if it only introduces new features but is backward compatible, and the TINY version if it's a bug fix or the like.
That said, if you are hosting via GitHub only, you can simply change the gemspec version to your preference as well as add a tag to that commit. Adding a tag makes it easy to find the correct version in GitHub after you've moved on to other versions - if using Bundler, for instance, your Gemfile entry would look like this:
gem 'your-gem', git: 'git://github.com/your-repo/your-gem.git', tag: 'v0.1.0'
You could also use the following (lengthy) set of commands to do this without bundler:
git clone git://github.com/your-repo/your-gem.git
cd your-gem
git checkout v0.1.0
gem build your-gem.gemspec
gem install your-gem-0.1.0.gem
If you intend to host via http://rubygems.org, you'll need to make yourself an account and then push your new version up to there. Rubygems will detect the version from the gemspec and provide it using the more standard Gemfile entry format:
gem 'your-gem', '=0.1.0'
Or using the gem
command:
gem install your-gem -v '=0.1.0'