Github has a simple web interface that makes it very easy to propose a simple change to code. You do not need to install any software or do anything beside register for a git hub account.
These suggestions apply to forks of both joomla-cms and joomla-platform, although the screenshots are for joomla-cms.
Note if you want to make the change to 2.5 the branch should be 2.5.x, and if you want to make the change to 3.x then make the change to the staging branch. You should never PR to the master branch.
NOTE: For pull requests for joomla-cms, the 'base branch' on the left must be set to staging (not master).
NOTE: Joomla Code has now been deprecated. You do NOT need a JoomlaCode link in your pull request anymore.