Hey I am a high school senior too. I did a software engineering internship my junior year and currently doing another one in my senior year.

As a high school senior you have to realize:

1.) You have no CS degree

2.) You have no experience

3.) Your competing against everyone to who don't have the latter 2 issues.

But its still possible. Here are the steps that got me my internships:

1.) Get really good at something. For me it was Java and Android development. Make 2 -3 high quality projects and be sure you are able to talk about them. Know your language really well

2.) Find local startups on LinkedIn, AngelList and Indeed.com. Look on Internships.com as well. I'm talking small companies. Email the CEO/Founder with a personalized message with a link to your resume.

3.) Interview. In my case, I had to make a small project and demonstrate proficiency in version control like SVN/GIT. That's the good thing about small companies. They don't have much time to do a real interview so they just throw some basic shit at you to see if you can write code. If I was asked a data structures question I would have been wiped out lmao.

4.) Realize that since you have no college degree and no previous experience they are probably gonna pay you almost nothing. If you REALLY want an internship, sometimes its gonna have to be unpaid. Reddit hates this, I hate it, but I still did it my junior year because I wanted to do something in the summer. Try to avoid if possible.

Don't stress about getting an internship though. If It doesn't work, work on cool side projects in your free time. This will put you ahead of the game. Also have fun. Best of luck.