Congratulations! I am 28+5 weeks pg with my first. Unfortunately there's not much you can do about the tiredness. The first 12 weeks were completely knackering, I was like the walking dead! The good news is that it does get better, or maybe you just get used to it.
I have been trying to keep it at bay by avoiding junk food and eating lots fruit/veg, healthy carbs & lean protein. Putting on excess weight will put further strain on your body & make you more tired so try to do some gentle exercise every day. I walk to and from work (1 mile each way), try to go swimming once a week and use the weight machines at the gym on the lowest weight setting.
Being conscientious about weight-gain will also help to minimise stretch-marks but there really is nothing you can do to stop them. I haven't got any at the moment but I read that they normally strike at about 30 weeks so I've only got a week left! Just use a moisturiser to keep your skin supple and take it easy.
Good luck