I would suggest you to go through the following resources in order to get yourself acquainted with Bootstrap
Read this StackOverFlow entry of the question “What is bootstrap”.
Microsoft: DEV203x Introduction to Bootstrap :- This course is extremely beginner friendly, Christopher Harrison would walk you through all the basic elements of the framework like Page Layout, Forms etc.
Halen Stevens “Bootstrap: You’re Doing It Wrong” :- This blog post highlights some of the best practices that you should follow in order to produce beautiful websites.
Lastly, you can always access the Official Bootstrap documentation to know more about it’s features.
These are some of the best resources I found, read them in the sequential order.
Wikipedia’s entry of HTML and CSS :- Read what these words mean, their applications in the real world.
CodeCademy’s HTML/CSS track :- Hands down, it is the best way anyone could get comfortable with the basic structure and syntices.
Shay Howe’s Tutorials :- Though this may come as a little overwhelming, this is by far the best series for diving deeper into front end development. Go slow, take your time.
If you don’t understand much from Shay Howe’s Tutorials, you can use W3CSchools and TutorialsPoint (HTML, CSS).
At this stage you are ready to create some cool project of yours, go design a webpage, imitate one!