Empieza is the conjugated form of the verb empezar for él/ella/usted. Here, the subject is la reunión, which is third-person singular, so you need empieza.
Also, empezar is a stem-changing verb in the present tense: e → ie in stressed forms.
So la reunión empieza is grammatically correct.
Spanish often uses the present tense for scheduled future events, just like English does in sentences such as The meeting starts at seven. So La reunión empieza a las siete is a very natural way to talk about a future timetable or arrangement.
You could also say La reunión va a empezar a las siete or La reunión empezará a las siete, but the simple present is very common here.
Spanish uses the preposition a with clock times, where English uses at. So:
So a las siete is the normal way to say at seven.
Yes. Spanish word order is more flexible than English word order. La reunión empieza a las siete is a very neutral sentence. Empieza la reunión a las siete is also correct, but it may shift the emphasis slightly.
For example, Empieza la reunión a las siete can sound a bit more like you are focusing on the fact that it starts at seven.
The written accent in reunión shows that the stress falls on the last syllable: reunión. In Spanish spelling, a word ending in n, s, or a vowel would normally be stressed on the second-to-last syllable unless an accent mark shows otherwise. The accent tells you not to stress it that way.
A rough pronunciation is reh-oo-NYON, with the stress on the end.