If Word thinks that you have mis-spelt a word it will underline it in red. To correct this error (if it is an error), right click on the  word. For example if I spell "magnitude" as "magnitide", a red line will appear under the word. If I right click on the word I will see a pop-up screen similar to the one on the right.

To correct the word I simply click on the correctly spelt word in the pop-up screen.

After you have completed a document and before you save it for the final time, you can conduct a spell check of the whole document.

Click on Tools
Click on Spelling and Grammar

Notice the icon to the left of Spelling and Grammar, which you could use instead of using the Tools menu.  

The spell check pop-up screen will identify the incorrect word in the top box under Not in Dictionary: and offer you possible correct spellings in the bottom box under Suggestions:.

To choose the correctly spelt word, click on it in the bottom box and click on the Change button to the right.

If the word is correctly spelt, but not recognised by Word, you could click on the Add button to add it to Word's dictionary. To proceed without changing the spelling you click on the Ignore button.

If the word is incorrectly spelt, and Word does not offer the correct spelling, click on word in the Not in Dictionary box and make the correction yourself. Then click on Change.

To end the spell check before it has proceeded to the end of the document, click on the Cancel button.

Thesaurus - finding alternative words

Click on Tools
Click on the double down arrow at the bottom of the menu
Click on Language
Click on Thesaurus

Thesaurus will offer you a list of words with similar meanings on the left hand side of the pop-up screen.

Choose the meaning you want e.g. size (see below)
You will then see a list of other words that could be used instead of the word magnitude on the right hand side. Click on the word you would like to use instead of the one you typed. For example, if I want to use size instead of magnitude, I would click on size in the right hand column and then click on the Replace button.