I took Spanish in high school. How do I get it back?

Start by listening to and reading easy Spanish to wake up what you already know, then fill the gaps and begin speaking. Your high school Spanish isn't gone, it's buried, and it usually comes back much faster than it took to learn the first time. Expect a few weeks of rust before it starts feeling familiar.

What you probably still have

Even if you think you forgot everything, you likely kept more than you realize:

A decent base of vocabulary you can recognize, even if you can't produce it
A feel for how sentences are built
The basics of present tense verbs
Pronunciation habits that are a good starting point

What usually fades is speed. You know the word cocina when you see it but can't pull it up while talking. That's recall, and it recovers with use.

Why high school Spanish often feels useless

Honestly, a lot of school Spanish was built around passing tests. Vocabulary lists, conjugation tables, fill-in-the-blank. You may have learned about Spanish without hearing much of it. That's why many people say "I took four years and can't say anything."

That's useful information. It tells you what to change: less table memorizing, more listening and speaking.

A rough plan to get it back

Week 1 to 2: just listen and read. Easy stories, slow videos, simple texts. Don't try to produce anything. You're waking the vocabulary up.
Week 2 to 4: patch the basics. Review the present tense of common verbs (ser, estar, tener, ir, querer), plus the basics of past tense. Do it quickly, not as a course restart.
Month 1 to 2: start speaking. Talk to yourself, shadow audio, find a language exchange partner. This is where fluency starts, and it's the thing school mostly skipped. See practicing speaking by yourself.
Ongoing: lots of input. Keep listening to material at your level, and save words you meet in real content.

For those first weeks of easy listening, I'm biased since I made it, but Langtok has short Spanish stories you swipe through with tap-to-translate on every word, which suits waking up old vocabulary.

Where you probably are

Many people who took 2 to 4 years of school Spanish land somewhere around A1 to A2 after the rust is gone. If you were more serious, maybe B1. A quick check is in how to figure out your level.

Don't be discouraged if you test low. Remember that you're starting with a head start, and you'll move through A1 and A2 more quickly than a true beginner.

What to skip

You don't need to redo your old textbook. Skip the chapters on topics you already know. Skip long grammar drills. Spend your time on understanding real Spanish, since that's the skill that was probably missing.

Mindset

Try not to feel embarrassed that you "should" know more. Most people who took school Spanish are in the same position. Give yourself ten to fifteen minutes a day, and in a couple of months you'll be past where you ended in school.

My take: this is one of the best starting points for a learner, because you already have a foundation. You mainly need exposure and some confidence. If you've also had a longer break from everything, see getting back into Spanish after a break.