Overwhelmed with Rome... Need Help!

Hi glory!
I’m traveling this Fall and have purchased most of my tours thru Viator; either small group or skip-the-line for Sistine and Borghese in Rome.
It’s only my second time ever buying Tour tickets, but my first (Iceland February 2018) was a very positive experience.
Safe travels!

gloryrae8, the "best" sites to buy tickets from are the ticketing sites for the attractions themselves. Only the attractions with the longest lines benefit from advance ticketing, and those are few: mostly the Vatican Museums and the Colosseum.

Vatican Museums: "open" tickets (general entry) or tours with a guide can be purchased from the site linked below. You choose an open time slot to skip the ticket lines but everyone has to pass through a security check. That queue moves quickly.

You can skip the line at the St Peter's basilica by booking a guided tour of the museums or the museums/basilica. Tour participants are allowed direct entry down a back passage reserved for that purpose:

http://www.museivaticani.va/content/museivaticani/en.html

Colosseum/forum/Palatine (a single "open" ticket covers all three):

https://www.coopculture.it/en/colosseo-e-shop.cfm

Do be aware that you can't enter the Colosseum before 2:00 PM on an "open" ticket: you must book a ticket with a scheduled-entry time earlier in the day if you want to get in before the afternoon. That ticket also covers the Palatine and Forum.

Reservations for Galleria Borghese are mandatory and sell out quickly. Make your reservation here; you will have to choose an open 2-hour time slot.

http://gebart.it/en/home_en/borghese-gallery-and-museum

Depending on when your trip is, it may be too early to order tickets for some of these attractions. You don't mention an interest in tours for the attractions above so you'd need to be specific if that's what you're looking for? The coopculture tours of the Colosseum Underground and Belvedere are notoriously difficult to land and often sell out within minutes of the hour they becoming available.

From my personal experience-

Buy the small group tour for the Colosseum as the background and insight provided by the tour guide will probably be well worth it. Plus, if you want to see the lower and upper levels, I believe (but I may be wrong) you need to be on a tour. You will be released from your tour inside the Colosseum and you will be free to wander around for as long as you like. However, if you have bundled your Colosseum tour with the Forum, you will have an additional timeline to meet and that may cut short your visit. We, however, had time for a quick lunch in between our two (bundled) tours.

If you feel you need the tour guide's monologue, by all means get the small group tour of the Vatican. But...

The main hall in the Vatican Museum is 700 meters long and jam-packed with antiquities everywhere you look and the tour guide will only cover the highlights. To put that into context, it's nearly a half mile long! Plus, when we were there (for an early morning tour), the hall was a continuous stream of people nearly shoulder to shoulder and in near lock-step the entire length of the main hall. In the Sistine Chapel, silence is rigorously imposed so your tour guide will not be providing any narration. The usually accompany you in with instructions to meet outside the exit at an appointed time. You are then taken to St Peters for a quick orientation and then be released.

If you think you will want more time and more information, you might consider skipping the tour and purchasing "skip the line" tickets and and renting an audio guide device.

Get your combo ticket for Palatine Hill/Forum/Coliseum early in the morning from the less crowded Palatine Hill ticket office. Make sure every member of your group is with you as they will only sell you as many tickets as there are people with you. When you go through the Sistine Chapel go through the exit door marked for tour groups and you’ll be able to go outside and enter St Peter’s without going through their security line. These are described in the guidebooks as well.

Oh and I’d definitely buy the Vatican Museum tickets online in advance.

When you go through the Sistine Chapel go through the exit door marked
for tour groups and you’ll be able to go outside and enter St Peter’s
without going through their security line. These are described in the
guidebooks as well.

But this is only a certain option IF you've booked a tour. Some independent tourists have been turned away by the guards, and it's up to whomever is on duty whether they will or won't allow access to non-tour visitors.

if you want to see the lower and upper levels, I believe (but I may be
wrong) you need to be on a tour.

Yes, that is correct. If unable to snag one of the coopculture tours, there are outside companies which offer tours - albeit for a higher price - as well.