It is normal to have Git questions when working in coding. If you are willing to learn how to use Git, I must recommend you to read Pro Git book but it is a long read. In case you are searching for quick tips or a lifejacket, take a look.
These are the most important tips and the most frequently asked questions.

How to sign a commit with “Signed-off-by:”

A lot of projects request to their contributors to sign their commits, e.g. Linux kernel. This is used to track how have wrote, reviewed or merged the commit. In first place you will need to configure your email and name.

git config "Your name"
git config youremail@domain.whatever

In addition, you can configure it for globally (for all your projects) adding --global to the command. Then, you can sign the commit.

git commit -s

How to modify the last commit?

After a review, you will need to modify your commit adding or removing code. In order to do this you should amend the changes. After adding the changes in staged you can amend the changes to the last commit. This will allow you to modify the commit message too.

git commit --amend

How to undo a wrong amend?

I have failed miserably many times to make amends when I should not have. So I spent some time investigating how to fix them without needing to re-do all the work.

git reset --soft HEAD@{1}

Here is the full explanation. This is moving the current head so it is pointing at the old commit but leaving the index intact for redoing the commit. This works because HEAD@{1} is the commit that HEAD pointed at before it was moved to where it currently points at.

How to modify the commit order?

If you need to modify the order of the commits you can use an interactive rebase. The number indicates how many commits you want to include in the rebase, if you pick two, you will get the two more recent commits.

git rebase -i HEAD~2

This will show you something like this.

pick bef571b second commit
pick 3099402 most recent commit 
# Rebase a7732d0..3099402 onto a7732d0 (2 commands)                             
# Commands:                                                                     
# p, pick <commit> = use commit                                                 
# r, reword <commit> = use commit, but edit the commit message                  
# e, edit <commit> = use commit, but stop for amending                          
# s, squash <commit> = use commit, but meld into previous commit                
# f, fixup <commit> = like "squash", but discard this commit's log message      
# x, exec <command> = run command (the rest of the line) using shell            
# b, break = stop here (continue rebase later with 'git rebase --continue')     
# d, drop <commit> = remove commit                                              
# l, label <label> = label current HEAD with a name                             
# t, reset <label> = reset HEAD to a label                                      
# m, merge [-C <commit> | -c <commit>] <label> [# <oneline>]                    
# .       create a merge commit using the original merge commit's               
# .       message (or the oneline, if no original merge commit was              
# .       specified). Use -c <commit> to reword the commit message.             
# These lines can be re-ordered; they are executed from top to bottom.          
# If you remove a line here THAT COMMIT WILL BE LOST.                           
# However, if you remove everything, the rebase will be aborted.                
# Note that empty commits are commented out        

Changing the order of the commit there, will allow you to change the order of the commits.

How to modify a commit that is not the most recent one?

If you need to modify a commit that is not the most recentm then you can use an interactive rebase. Please, do it exactly as I did above but instead of modifying the commit order, replace pick with edit in the commits that you want to edit.
This will allow you add/remove changes to the commit and then save them doing the following command.

git rebase --continue