Can I make sure that only the first letter in my text fields is capital?

Yes - you can.
It is possible to add a value rule for this using REGEX, and the form filler will then be forced to enter the field according to your expression.
Read more about how to use a value rule with REGEX.

Following are steps:
1- Create a simple Salesforce Table:
2- Drag a textbox
3- Select it on canvas
4- Create a value rule and choose REGEX
5- Enter the following:   /^[A-Z][a-z0-9_-]{3,19}$/
6- Save the form