Hey Kelsey - you should start by googling CSS basics, as most (if not all) of these types of questions can be answered by tooling around on a few good resource sites (like w3schools.com).
If it's on a landing page, you may just need to update your style sheet (look for the H1, H2, H3, etc. headings within the <style> tag in the header or linked style sheet). If it's in-line in an email, you can try the WYSIWYG, or edit the HTML directly. If it's something you want to adjust for all emails, edit the template.
And if that's all greek to you and you have no desire to learn the essentials about HTML or CSS... Hire a consultant just for some template creation. Email Monks does great work, fast, and you'll never have to edit a single line of code ever again. Other consultant shops like Fathom, LeadMD, Digital Pi, et al can also do the work, but they're all more interested in partnering on longer term strategy. There are also a number of champs that freelance here (Juli James?)