Welcome back to marijuana! As other commenters have said, any dispensary will be able to help you. If you just walk up to the counter and tell them what you said to us, they will be able to suggest a strain that would suit you best.
There are two major strains of marijuana- sativas and indicas. Sativas are more of a "daytime high", it relaxes you a bit but you can still get stuff done. Some people find that sativas make them a bit more paranoid. Indicas are more of a sleepy "night-time high". There are also a lot of hybrid strains which have qualities from both. I prefer Blue Dream or Dream Lotus (the second is a plant that was derived from the first) as it calms me down and helps my depression and anxiety.
As for what you should expect, it isn't that complicated. Everywhere I've been has had a small waiting room with a window. You go up to the window, show your ID, then are brought back into a much bigger room where they keep all the product. At the counter, the budtender will ask what you're looking for- if you don't know the exact strain you want, just tell them and they will help. I've run into a couple of budtenders who didn't really know the difference between the strains, but each time they immediately grabbed someone who knew more. Once you decide on a strain and a quantity, they will measure it out in front of you. I would suggest buying a gram your first time- it's a very small amount, so if you don't like it, you're not out much.
If the dispensary doesn't sell pre-rolled joints (it should say on their menu if they do) you will need something to smoke with. Most dispensaries will have glassware. The place I go on Front Street has a huge basket of $3 glass pipes. If you decide you want to get more serious about smoking, you should invest in a bong/bubbler/vape or whatever, but you don't really need anything super fancy if you're just trying it out. Honestly, if you're desperate, you can McGuyver a bong out of just about anything- hell- I once saw someone smoke using a butter knife and a stove- but glass pipes are plentiful and cheap. Obviously you also need a lighter.
If you're going to smoke using a pipe, be sure to break up the marijuana first- don't just put a whole nug in the bowl piece and light it on fire. Breaking it up into "shake", or small pieces, ensures even burning and a better smoking experience. Take a SMALL hit your first time, it will burn. Have a bottle of water handy. Also keep in mind that this marijuana is likely a LOT stronger than what you used 15 years ago. After the first hit, wait a bit- you can always smoke more later, but being TOO high can be pretty awful.
The prices on the menu are for medical customers- recreational customers are required to pay a 25% sales tax on top of the price listed. Most dispensaries have an ATM on site, but be warned, they do not take credit/debit cards or checks. Since marijuana is federally still illegal, dispensaries are in a legal limbo and banks won't work with them. Have cash. They will ring you up like any other store. If your budtender was professional and helpful, consider tipping them- most have tip jars.
If you're a very occasional marijuana user, it should be out of your system in about three weeks. If you start using regularly, it will take longer. Certain combinations of vitamins and water will help speed up the process, and you can buy kits to fool the tests as well if you're worried.