ごはんとにくとたまごとレタスおたべました。 This sentence isn't wrong, but it implies that you ONLY ate rice, meat, eggs, and lettuce.

ごはんやにくやたまごやレタスをたべました。 By using the particle や it means "I ate such things as rice, meat, eggs, and lettuce" and implies that you ate other things besides that. It's useful for making lists or just if you want to give the person you are talking to an idea of what you did.

たべます！

今日のひろごはんに私はレストランにいきました。
よにんのともだちがレストランに私と一緒にいきました。
ごはんとにくとたまごとレタスをたべました。
すべてがおいしかった！

I don't know if I'm speaking like a robot or a book, so please, fell free to correct me if you want.

The parts in red are commented below.

*Using 私 too much when it's obvious in the context that the information is about you sounds very English-speaking-Japanese. A more common sentence would be 今日のひるごにはんはレストランへ行きました。　４にんのともだちといっしょにいきました。