Well first I am sorry about your loss.
bar_enosh is correct in the Order of the Builders was a youth organization begin in 1921 and sponsored by the Masonic Fraternity. It is very similar to the DeMolay organization. Being a member of these organizations does not automatically make you a Freemason when you reach the age of majority.
Whether your grandfather was or was not a member of the Masons really doesn't matter. The Freemason will consider any man who meets the criteria. The catch is you must ask to join, they are barred from asking anyone to join.
The reason is one must come of their own free will to Masonry.
So in most US jurisdiction the criteria for membership is:
Be a man, freeborn, of lawful age (18-21 depending on the state you live in) and well recommended. generally this means good character (no criminal record and have a good reputation as a citizen).
Also have a belief in a Supreme Being and the immortality of the soul.
Interesting that you say your grandmother was in the OES as this organization only accepts women who are directly related to Master Masons, i.e. wives, sisters, mothers, daughters of Master Masons. So for your grandmother to have joined, either her father, brother, husband would have to been Master Masons. I would look into that as it may prove quite interesting to you.
Anyway if you meet the criteria of the Lodge in your jurisdiction, you can just ask for a petition and off you go.
One way to get started is to look up your Grand Lodge, The grand Lodge of (your state or country here), keep in mind in the US there is no central Masonic authority for all states, so each state has a Grand Lodge. The Grand Lodge websites usually have Lodge locators and you can find a Lodge that way. Many local Lodges now have websites, FB pages, etc and announce their events, like BBQs and other fundraisers. You could find out when the local Lodge is meeting or having one of these events and attend. This would allow you to meet some of the members ina relaxed social setting and for them to meet you and get to know you.
Notes:
Design by G_d does not know what he is talking about and is typical of religious ignorance. Freemasonry is not a religion, not a substitute for one, although religious in nature. It teaches not religious dogma and will not interfere with a members chosen faith. So naturally the Lodge does not evangelize like the fanatics would like. In fact the discussion of religion and politics is barred in a Masonic Lodge as these are subjects that would divide men and keep them at a perpetual distance.
Also you will never me invited as you must ask.
Vulcan must still be on Vulcan because the Lodge membership in some areas is actually expanding a bit and in most holding steady. There wa a great influx of members after WW2 and the membership roles swelled to abnormal highs. Well now as you imagine many of these members of the greatest generation are passing on and membership numbers are returning to normal. So it may seem like a contraction, but reality is it's just normal ebb and flow.
Good luck.