Consistent crashing on multiplayer and single player servers

java version
I am currently experiencing issues with connecting to my minecraft multiplayer servers and every time I connect to a server, the game seems to crash so I have about 3 seconds to approximately enjoy.
Also on multiplayer servers, such as cube craft.net I am aware that other players can chat to the whole team whilst when I search for help tutorials, it will only allow me to send messages to an individual player. I would like to know how to speak on team chat and also global chat.
Please may you help me with my problems???