How to switch to other branch in Source Tree to commit the code?

Hi I'm also relatively new but I can give you basic help.

1. To switch to another branch use "Checkout". Just click on your branch and then on the button "checkout" at the top.

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UPDATE 12.01.2016:

The bold line is the current branch.

You can also just double click a branch to use checkout.

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2. Your first answer I think depends on the repository you use (like github or bitbucket). Maybe the "Show hosted repository"-Button can help you (Left panel, bottom, right button = database with cog)

And here some helpful links:

Easy Git Guide

Git-flow - Git branching model

Tips on branching with sourcetree