You can usually sketch examples of what you mean. When you give instructions for a lab, go through the motions of the lab while you are explaining itand then do it again, more quickly, to make sure everyone understands. If you don't know the English word for something, but you manage to get the idea across and the students don't know the English word either, teach them the word in your language. Afterwards, pause whenever you use it, to make sure they remember what it means. Tell them the correct English word in the next class.
Teaching takes patience.
Have models available whenever you can, and consider using the students themselves to model how something works. As a TA, don't think of yourself as a person with authority over your studentseven if you have some. Try to think of yourself as a helper and friend. If you are willing to step out of your shyness and do all you can do to communicate with them, they will really appreciate it, no matter how good or bad your English is. A good philosophy to remember is, who cares how you say it, as long as you communicate.