Сегодня вечером мы встретимся у входа в театр.

Because встретимся is from the perfective verb встретиться. In Russian, perfective verbs do not have a true present tense, so forms like this are understood as simple future.

• мы встретимся = we will meet
• compare мы встречаемся = we are meeting / we meet regularly

Here the speaker means one meeting that will happen later, so встретимся is the natural choice.

The ending -ся makes the verb reflexive, and with verbs of meeting it often gives a reciprocal meaning: meet each other.

Compare:

• встретить кого-то = to meet someone, to encounter someone
• встретиться с кем-то = to meet with someone / to meet each other

So мы встретимся means we will meet each other or we’ll meet up.

That’s true: Russian often omits pronouns when the verb already shows who is doing the action.

So both are possible:

• Сегодня вечером мы встретимся у входа в театр.
• Сегодня вечером встретимся у входа в театр.

Including мы can add a little clarity or emphasis, especially if the speaker wants to stress we rather than someone else. Without мы, the sentence still sounds completely natural.

In Russian, parts of the day are often used in the instrumental case to express time when something happens:

• утром = in the morning
• днём = in the daytime
• вечером = in the evening
• ночью = at night

So сегодня вечером literally looks like today, in-the-evening, but it simply means this evening / tonight.

The preposition у takes the genitive case and often means by, near, or at.

So:

• вход = entrance
• у входа = by the entrance / at the entrance

Here входа is the genitive singular form of вход.

Because the phrase is вход в театр, which means entrance to the theater or more literally entrance into the theater.

The preposition в here keeps its directional meaning, so it takes the accusative case:

• в театр = into the theater

If you said в театре, that would mean in the theater, referring to location inside the building, not the idea of an entrance leading into it.

Also, театр is an inanimate masculine noun, so its accusative singular looks the same as the dictionary form:

• nominative: театр
• accusative: театр

У театра means by the theater in a general sense — somewhere outside or near the building.

У входа в театр is more specific: by the entrance to the theater.

So if you want to make the meeting point clearer, у входа в театр is better.

Yes. Russian word order is quite flexible.

This sentence:

• Сегодня вечером мы встретимся у входа в театр.

is natural and emphasizes the time first: as for this evening...

You could also say:

• Мы встретимся сегодня вечером у входа в театр.
• У входа в театр мы встретимся сегодня вечером.

The basic meaning stays the same, but the emphasis shifts depending on which part comes first or last.

Russian does not have articles like a and the.

So театр can mean the theater or a theater, depending on context. In this sentence, it will usually be understood as the theater because the speaker and listener presumably know which theater is meant.