Use "I guess..." to express that you've just started to think something. For example, you can use it when:

• you read the instructions for something, and now you think you can explain how to do it
• a friend has been trying to convince you to go to a party, and you've finally been convinced to go
• you've been waiting for a long time for someone to call you, and now you've decided that they're not going to call.

Here are some examples:

Hmm. They're not here. I guess they got tired of waiting.

I guess I might as well stick around and have one more.

I guess so.