I am SOO tired, advice please!

I'm 6 weeks pregnant and I have absolutely no energy. I've been sleeping 12-15 hours a night and the whole time I'm awake I feel like I could go back to sleep. Walking upstairs is like climbing a mountain. If I go out to get groceries or to visit family or anything really, I come home completely drained. I'm not exactly fit but I've always been fairly healthy. Any tips to boost my energy or is this just a phase of the pregnancy? This is my first time being pregnant and I'm just worried that my whole pregnancy I will be this exhausted.