Pere Lachise

It will not be hard at all. It also depends on where you meet up with your tour? If you are doing the Paris Walk tour it meets at the Pere Lachaise Metro stop, exit blvd Ménilmontant. It that is not the tour you're on, let us know where the meet up is.

Here's how you do it.

Walk to the Ecole Militaire stop on Line 8. Go in the direction of Creteil.

Take Line 8 to the Opera stop.

Change at Opera to Line 3 in the direction of Gallieni. You'll go 10 stops to Pere Lachaise stop.

https://www.ratp.fr/itineraires/Ecole%20Militaire_%2075007%20Paris%26P%C3%A8re-Lachaise_%2075011%20Paris

Will you already have been in Paris a few days? You'll need a couple of Metro tickets for this ride - one going and one coming home. You can also take the 69 bus home from the Cemetery and pass a bunch of famous sites.

editing to add: And honestly, just ignore snotty remarks about the hotel location. I've stayed there, enjoyed the hotel and location. I know you'll enjoy the Best of Paris tour! Who's your guide?

Bus 69 from the Rapp - La Bourdonnais stop near you might be a good option. The route offers so good site seeing along the way.

ask your hotel or use your smart phone to take the 69 bus it is way more scenic.

There are 5 entrances to the Père Lachaise cemetery.

The main entrance is on the "boulevard de Ménilmontant" next to the Philippe Auguste metro station, line 2. if this is where you will meet your guide.

The second major entrance is next to the "Place Gambetta" . metro station "Gambetta" - line 3

I echo the votes for the 69 bus. That's what we took to Pere Lachaise after our RSE Paris and the Heart of France tour in 2017. We followed Rick's self-guided tour from his book and saw everything we needed to see and more.

I do NOT recommend the bus anymore. Service quality on Paris city buses is much worse now than pre-2019, and the trip to Gambetta on Line 69 will probably involve a 10-minute wait for the bus followed by a 50 to 60-minute crawl through the traffic of the streets of Paris. Not fun.

I’d take the métro there (so you aren’t late) and the 69 bus back for the sightseeing.

I took Rick's infamous 69 bus in December to Gambetta and it was just fine. I picked it up at Bastille because I was in that area.

Google maps isn't perfect but it shows Bus 69 is taking only a few more minutes than any metro route. I'll take 48 minutes above ground over 45 minutes below ground any day.
Also -- I'm sure the OPs guide knows best and determines the meeting spot, but Rick's advice about starting on the the cemetery tour uphill/Gambetta/north side and working down is gold.

Back in 2015 we took the metro to the cemetery. I thought it was pretty straightforward getting there.

A bus or taxi can take forever in central Paris. If you want to be on time take the metro. Any site that shows the time being similar to metro is not including traffic -- we have had it take an hour from St. Lazare to Gare d'Lyon by cab -- never again.

Another vote for Bus #69. I always prefer buses to the metro when in Paris. They are simple, no stairs, and let you actually see the city and its neighborhoods en route rather than tunneling underground.

Lots of individual opinions and preferences are built into in these recommendations. Best advice here, in my opinion, is to do it the way that suits you best.
You got excellent detailed advice from Pam on taking the metro (and that's what you started off saying you intended to do).
I am not among the advocates of Bus 69. We took it to Père Lachaise and back and were unimpressed. Just a bus ride through a big city. If you've done that before, this one won't seem very special in my opinion. Of course, the big city in this case is Paris, so there's that, but it's a regular city bus, not a sightseeing tour.
Taking a taxi is easy and more expensive than bus or metro.
You say it's your first day in Paris. Will you be arriving from someplace else in Europe? Straight off the plane? Might you be jet-lagged? That's a very real phenomenon and should be taken into account.
Best of luck to you.

There are several entrances to Père Lachaise. Your tour meet-up is at the Alexandre Dumas Metro Station, not the Père Lachaise station. It’s a bit of a walk between the two. If you are unaccustomed to public transit, and arriving for the first time in Paris, your initial idea of the taxi could be the easiest. Leave plenty of time to get there, to allow for traffic. Then sort out the metro for your return.

It will be easier to take a taxi but it is a relatively long trip, probably 30 minutes and cost anywhere from 15€ to maybe around 35€, depending on traffic. Ecole Militaire is on Line 8 is the metro stop nearest to your hotel. If your tour is meeting up at Alexandre Dumas metro station, you will need to get to line 2. Unfortunately Lines 8 and 2 do not connect so you would have to make two transfers. Or you can skip the transferring and wandering about underground and exit Line 8 at the Faidherbe–Chaligny metro station and walk about 20 minutes. That's what I would do or actually more likely, I would ask Cedric (if he still works at the hotel) to call a taxi for me. It's a very simple walk. Exit the metro at the Rue Faidherbe exit (Exit 1). Walk north/northwest on Rue Faidherbe (toward Rue Paul Bert) until you get to Rue de Charonne and turn right and continue on Rue de Charonne to the Alexandre Dumas metro station, which is at the corner of Rue de Charonne and Boulevard de Charonne. There is a slightly shorter way up Rue Paul Bert to Rue Jules Valles to Rue de Charonne but I can't really describe it and it only saves about a minute.

Thank you for coming back to edit your original post and add in your meet up location!

I’ve been looking at a Paris Walks tour on April 15, lol. I’m sure you’ll enjoy it….it’s a very cool cemetery!

One more thought…another route by Metro (just slightly longer walk than to École Militaire), would be to walk up Avenue Rapp, cross the Pont d’Alma and take Metro Line 9 from the Alma Marceau station to the Charonne station (Walk 5 blocks east on rue de Charonne and there you are at Alexandre Dumas station. These are actually both pleasant walks and avoid changing trains. I would always rather walk above ground than trail through the Metro tunnels to change trains twice.

Wish I had thought of this sooner when I recommended the taxi to Alexandre Dumas.