Dice the raw shrimp into small bite-size pieces and add them into a large non-reactive bowl. Glass or food-grade plastic bowls are both good options.
Add the lemon juice, lime juice, red onion, garlic, and jalapeño to the bowl and lightly toss together with the shrimp to combine.
Cover and refrigerate for 20 minutes until the shrimp is bright pink and opaque.
Add the tomatoes, cucumber, and cilantro to the bowl. Lightly toss to combine, cover, and refrigerate for an additional 10 minutes.
Add in the avocado, salt, and black pepper. Lightly toss to combine, taste, and add more salt as needed. Serve immediately with tostadas or tortilla chips.
Notes
If using cooked shrimp, toss together all ingredients minus the salt, black pepper, and avocado. Let marinate in the fridge for 15 minutes. When ready to serve, add salt, black pepper, and avocado.