I would start with very structured activities, which need only simple English, which are preceded by a clear model (this is verry important), and which in some cases they can prepare in advance, but which on no account mean giving a limited answer to one question from you.
For example, a questionnaire :
When you were small, what was your favourite toy? ....Did you spend much time with your grandparents? ... Were you frightened of anything? etc. In the first lesson give them the questions and some model answers eg
Yes, I was frightened of the dark which all have a
Yes/No + comment structure. They have to match the questions and answers. For homework they then have to prepare their own answers and in the next class, ask each other and answer the questions . Other questionnaires, depending on the structures they've covered include : present simple -
What do you usually have for breakfast? What do you do after school? etc; present perfect -
Have you ever eaten curry? Have you ever seen a lion? etc; and
would like -
Would you like to visit the States? Would you like to go into space? Would you like to be a doctor? Would you like to learn another language? etc. You can create a questionnaire for any structure they know.
Once they're doing these simple question/answer type activities happily, move on to more extended speaking activities, but always focused on something concrete and, at first, preparable. In another post someone suggested
Show and Tell. This would also be ideal as you can encourage them to keep the object very simple, but they can prepare before the lesson. Again, give a couple of examples yourself in the first lesson so they understand what you want and that it only has to be very simple. In all of these prepared activities, don't let them read out a prepared script, but for the first few times at least, don't worry if they appear to have memorised what they say. The important thing at this stage is that they are speaking in sentences and gaining confidence.
Once that confidence has been gained, but only then, move on to activities where they have to speak spontaneously - but still with something very concrete to talk about. Kim's Game, where you place a number of objects on the table, then cover them with a cloth and ask the students to remember what was there is one possiblity, or the very similar
Picture Memory Game.
These are only a few examples of the games and activity types you could use. You'll find lots more on this site and other sites which link to it. The principles to keep in mind are : keep it really simple at the beginning and increase the difficulty step by step; always provide the students with a model of what you want; and give preparation time wherever possible.