Which Titleist Clubs for a beginner?

There is no way we could recommend a fit for you just based on your age or skill level. You will be better off finding a Titleist fitter in your area and having them do a detailed fitting. If you are going to invest the money in a full set of new clubs, you probably want to get what fits you best, not the recommendation of a bunch of folks here that have never seen you play.

I would never recommend senior flex for you. I’m not an expert fitter and even though I have light shafts, I only know they are best for me.

Your coach should be able to find you or recommend to you about 5 used clubs to start with. Driver, a 4/5 hybrid, a 7 iron, a P pitching wedge, and a putter. It is really hard to fit someone that misses good contact half the time. That’s just where almost all of us started.

The technology is so much more complex than 2-3 shaft flexes. Titleist offers 4 drivers, 7 irons, and 30+ variations of wedges. The selection is so overwhelming you will best be served by seeking expert advice that can see your swing and find you the best option. Glad you have had one lesson. The best investment for you to start is coaching. A $500 driver will not be your solution to make the PGA Champions Tour. At least it hasn’t for me….. Once you gain some consistency, then by all means make a pro shop happy with a full bag purchase.

At 77 I would recommend seeing a fitter and checking out the Titleist line of clubs to see what best fits your swing. Being you just taking up the game do you want to INVEST in a new set of clubs as you learn the game. A fitter can recommend the correct flex for you. A lot of beginners buy clubs with little knowledge of shafts and heads and begin a love hate (with more hate) for their equipment and that is where the fitter can help you enjoy the game and save you money. When I started playing in my late 40's I bought clubs off the rack and knew what I wanted based on little knowledge of the equipment or the game of golf. Biggest mistake of my life. Spent many years experimenting with various brands looking for the perfect set. You might even want to hold off until you progress with you golf lessons. Titleist makes some great clubs and no two golfers swing the same and most do not have the same set up. Find a good Titleist certified fitter. Welcome to Team Titleist and wishing you the best. =)

T350 irons. Get fit for the shafts and specs. You might be fit into the T400. But the newer version of these irons are only in Japan.

For the woods and hybrids, TSR2 or TSR3. Get fit for the ball flight, shafts and set make up.

Go get fit and work with your fitter to determine the best clubs for your abilities. There are hundreds of head and shaft options available, none of which anyone here can make without seeing your game in action. If you want a baseline to start from, consider the T350's or T400s, as they are engineered to help slower swing speeds get the ball in the air and go far, and with maximum forgiveness. A fitting will help you get the right heads and shafts in your hands to help you on your way. I wouldn't get wrapped up too much in labels, "Senior" flex might be your ticket to another 30 yards off the tee! Go find out what is best for your game by getting fitted!

We can’t tell you what you need but get a fitting and it will tell all.

I'll be the outlier here.
If you've only had one lesson, I would hold off on a fitting, unless you've got a few grand burning a hole in your pocket. The last thing you want is to put out 2 to 3 grand for a set of clubs when there is a good chance that once you develop a reliable swing, they don't work for you. And a 7 iron ball speed of 100 mph is pretty good for someone in their 60's.
If you're still taking lessons, first step is talk to your instructor for some advice on a starting point. Try out a few clubs at the local shop or at the golf course to get a feel for what works right now, and then pick up a good set of used clubs. T350, T300, or 718 AP1 will be a good starting point for Titleist irons. The TS2 line is a good choice for driver/fairways/hybrids. All good clubs that will hold their value for a year of developing your swing.
Once you get comfortable with your swing, then by all means get on board with a true Titleist fitting. It's a great experience, and will be very beneficial to your game.