I'm new in LaTeX and I want to use normal font in theorem, lemma, definition and other math environments and just remark some word in italic. I want to preserve the style when I put $ $ but not in all the definition statement. Now I only can get all in italics. Here you can see my packages, also I hear recommendations about change this (for example to use \usepackage{amsthm}). Thank you.
\usepackage[utf8x]{inputenc}
\usepackage{pmgraph}
\usepackage[normalem]{ulem}
\usepackage[spanish]{babel}
\usepackage{graphicx}
\usepackage{hyperref}
\usepackage{tabularx}
\usepackage{float}
\usepackage{amsfonts, amsmath}
\usepackage{txfonts}
\usepackage{tikz}
\usepackage{multicol}
\usepackage{capt-of}
\usepackage{fancyhdr}
\usepackage[left=2.54cm,top=2.54cm,right=2.54cm,bottom=2.54cm]{geometry}
\usepackage{amssymb}
\usepackage{enumerate}
\usepackage{enumitem}
\usepackage{mathabx}
\usepackage{parskip}
\newtheorem{algorithm}{Algoritmo}
\newtheorem{definition}{Definición}
\newtheorem{proposition}{Proposición}
\newtheorem{lemma}{Lema}
\newtheorem{theorem}{Teorema}
\newtheorem{corollary}{Corolario}
\newtheorem{proof}{Dem.}
