How can I make it so that all the letters on a word are all capitalized yet the first letter is bigger than the rest?

The style is called "small caps" and is considered a font shape by LaTeX. Thus, scshape will turn it on, or a delimited form, textsc{...} is available. Naturally, you must make sure your font supports this shape.

documentclass{article}
begin{document}
textsc{Definici'on (Isomorfismo)} Sea $T:Urightarrow V$
end{document}

p.s. Other font shapes include italic (itshape) and upright (upshape).