Congratulations! You must be thrilled about being a grandma, but perhaps unsure how to approach the situation and how to be involved. You may feel like a bit of an invader into your son's relationship, and may or may not have a good bond with your daughter in law.
Grandparenting is a beautiful adventure, but it may be a little harder on moms of sons. Your best approach may be to talk directly to your daughter in law, and to tell her that you would love to be there for her, and to support her in any way she needs. At the same time, pushing yourself and your advice in unsolicited ways is probably a bad idea. Your daughter in law will doubtless appreciate it if you send her parenting books you like, and if you offer to buy gifts but ask what they need first.
Believe me, I've had too many friends complain about unwanted gifts from their mother in law to ignore this particular grandparenting tip. If you ask what they need, your son and his partner will both thank you for it later. That works for so much more than gifts saying that you are available, but without being pushy, is a great way to let the parents of your future grandbaby know that you truly care about them. Saying you want to babysit may be annoying, but letting them know you would love to, when they need it, is great. You could also offer to help with housework or cooking if you want to.