Monica,
Essentially there are three areas where you could want to do this.
If you want to change a page or a post, you may be able to do this within a blog post itself, either because the theme allows you to, or by changing to Text view and using some HTML code. You have two different views of a post “Visual” or “text”. They are in the top right of where you type the body of your post or page. Click the “text” one to change and allow you to enter HTML code.
For instance
<p style="text-align: left;"><span style="color: red;font-size:18pt"><strong>What you want to say here.</strong></span>
will change the colour and size.
You can just copy this into your post and play with layout eg text align can be left right or center, most of the main colors can just be entered eg red, white, blue, and any color can be entered using hex codes if you know how to do these, and size has lots of different things, but for starters just change the pt numbers.
If you want to change text outside of pages and posts, then you’ll be getting far more technical.
You theme may allow you to do this – look at dashboard>themes to see if you theme has things that you can change.
Otherwise you’ll need to go “under the hood”, and start changing code. This takes a little understanding, but you’ll either need to change some code in the themes CSS stylesheet, or (better) create a childtheme and make changes there. To learn how to do this, buy “WordPress web-design for dummies” – make sure it’s the web-design one, not the basic wordpress for dummies – and that’ll take you through setting up a childtheme and playing with css
