Summer internships are usually full time, whereas fall or spring internships can be full time or as little as 8-10 hours a week during the school year. Opportunities are always available. What opportunities might fit your needs depends on your major, skills, financial needs, personal desires, and where you are willing to go. Positions can be found worldwide. While some internships may be available only to juniors and seniors, there are also plenty of opportunities for freshmen and sophomores.
Internships are truly a case where the more the better.
You will gain skills and networking contacts that can often result in permanent positions. There are many forms of compensation, and money is only one of them. Think about what you are looking for and what is being offered. Evaluate each position carefully as to what you can expect to learn. Careful thought must be given to personal, academic and vocational objectives, logistical needs, and your short and long term financial needs.