This is the official website for Pompeii:
http://pompeiisites.org/en/pompeii-map/
This is the ticketing portal:
http://pompeiisites.org/en/buy-tickets/
The € 16.00 "full price" ticket for adults does not include a tour, and note that they're currently only selling advance ticket through end of February. You could also just purchase an entry ticket when you get there although in advance might get you in sooner during high and shoulder season.
Colosseum: the coopculture site provided above is the official one, and this is the page you need for an "ordinary admission" ticket:
https://ecm.coopculture.it/index.php?option=com_snapp&view=products&catalogid=ADAE8874-AE4B-55B9-02FB-0167A81C0247&snappTemplate=template3&lang=en
They aren't currently selling tickets beyond end of March but when May tickets become available, you will need to choose a specific day and entry time. You will not be able to access the arena floor or underground without a tour.
Herculaneum: just purchase tickets at the site. I wouldn't try to do BOTH Herculaneum and Pompeii in one day, and I'm not a fan of day trips to Pompeii from Rome. Use an online translator for the Herculaneum website below:
https://ercolano.beniculturali.it