Are those German sentences correct?
There's a lot of things, that we have to talk about.
Es gibt viele Sachen, davon mussen wir sprechen.
Could we start with the guitar playing?
Könnten wir mit dem Gitarre-Spielen anfangen?
What place should we pick?
Für welches Ort sollen wir uns entscheiden?

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The first sentence would be extremely hard to understand for any native german speaker.
"Es gibt viele Sachen, über die wir sprechen müssen" would be the correct way to phrase the sentence

The other two seem perfectly fine to me. ^^

Edit: Whoops, seems like I totally overlooked a mistake here. In the third sentence "welches" is wrong, "welchen" would be the correct form to use. Thank you so much for pointing that out DonHolgo, I should read more carefully!

Answer

You should totally post stuff like this in the notebook section where there's tons of tools like colors etc. to better correct text and help you with questions like this :)

Nie przegap okazji, by nauczyć się języka w zaciszu własnego domu. Przejrzyj naszą ofertę doświadczonych korepetytorów i zapisz się na pierwszą lekcję już teraz!