Find a photo of a cruise ship, any cruise ship, & you'll see a row of lifeboats along the side. As other answers, an "obstructed view" cabin is behind that row of lifeboats. You may be lucky & have a decent view between two lifeboats - or unlucky & the view is entirely obstructed by a dirty great lifeboat. If a view is important to you, you should have paid a little more & booked an "ocean view" cabin.
But "obstructed view" is much better than an inside cabin, which has no window, because you will have some natural light, you can tell whether it's day or night, what the weather is like, whether you're in port or at sea. And you can tell where you are on the ship - inside cabins can be dis-orientating. All windows are fixed - they don't open.
As Scrapnana's post, the recommended level of tips is half that of US & Italian ships. Nowadays, P&O have done-away with envelopes for you to give cash - instead they do the same as most cruise lines, they add the recommended tip to your on-board account, along with your on-board purchases such as drinks, excursions, dining upgrades, & so on.
You have the right to ask on-board that the tips be removed from your on-board account. Some folk do this so that they can tip personally (a bit pointless, because the tips are pooled amongst the staff). And some like your friends do it to save money. Most cruisers see this as cheapskating, but that's up to them. I suggest you see what the service is like, then decide for yourself what you want to do. Simply leaving the auto-tip in place avoids complications - and the daily amount is what you'd probably tip in a restaurant at home just for an evening meal.
White towels are provided in your bathroom.
Sunbed towels are also provided (on P&O they're available on deck) and you can take them ashore for the beach.
Toiletries nowadays are mainly just soap and a dispenser of shampoo/body gel. Perfectly adequate unless you have specific preferences.
If you're sailing from the UK, there's no luggage/liquid limits that would affect you, so you may prefer to take your own.
If it's a fly-cruise there are severe limitations on liquids in your hand-luggage, and you'll have a weight limitation on your checked luggage - around 20kg to 23kg so don't waste that allowance on big bottles of toiletries.
One major saving with P&O compared to other cruise lines - you are allowed to take as much alcohol aboard as you like, within reason, to drink in your cabin. Make good use of that dispensation for pre-dinner drinks etc.
All first cruises are great. Have a super one