$ brew install git # If we want to use gitk (git GUI), this way is very convinient. (however, I think SourceTree is better.)

# rails project
$ rails new app
$ cd app
$ git init
$ git add .
$ git commit -m 'first commit'

$ git checkout -b branch_name
$ git branch # list branches in the project

# do some changes under this branch ...  

$ git add .
$ git commit -am 'some thing in this brach'

# more about `git commit` 
$ git commit -am 'xxxx' # == git add . + git commit -m 'xxxx'
$ git commit --amend # reword commit message

# go back to master and merge it

$ git checkout master
$ git merge branch_name

# push to github 

$ git remote add origin repos_git_ssh_url
$ git push --all # both master and branches
$ git push origin master # you also can push master to remote only

# reset or revert to HEAD version (HEAD refers to latest commit stage)

$ git reset HEAD --hard # --hard is dangerous (delete unstaged files)
$ git reset HEAD --soft # --soft will go back and keep files staged 
$ git reset HEAD # will go back without tracking unstaged files
$ git revert HEAD # make HEAD stage as new stage

# reset one file

$ git checkout -- <file>

# conflick of rebase (merge is the same)

$ git rebase master # under second branch
# conflick ...
# modify conflick <file>
# then 
$ git add <file>
$ git rebase --continue
# done.

# To remove untracked files / directories do:

$ git clean -fdx
# -f - force
# -d - directories too
# -x - remove ignored files too ( don't use this if you don't want to remove ignored files)