I ask this because I need to decide whether to hire a circuit with a guide or not. I prefer to travel by myself, but I'm not sure if it will be too difficult to move around and get to visit the major tourist attractions without knowing any Japanese at all. Is it hard to find people who speak English? Are signs in train stations, buses, public maps, and indications usually in English? etc. Thanks!!
If you are used to travelling by yourself, you will be fine. In subways and trains they usually make announcements in English as well as Japanese. At the very least for stations where tourists would want to get off, but often for all stations.
If you go to the information counter or the guy sitting next to the ticket gates in the train stations you can get subway and train maps with English text. Staff is usually very friendly everywhere, and they will try to explain in English, even if that takes some time.
Buses can be Japanese only, but they are seldom needed to reach tourist attractions (depends a bit on where you want to go), and if you talk to the driver (try not to be there at rush hour) and tell them you want to get off at soandso and sit near them, they will usually get you to the right place.
It is also useful to visit tourist offices, the people there usually have English maps and flyers and can tell you how to get to places. They are usually in every bigger train station.
If you have to ask someone on the street, you should go for someone between 25 and 40 who doesn't look too busy. Their English might not be perfect, but they will almost definitely try to help you and send you in the right direction.
If you give me a list of where exactly you want to go, I can probably give a more detailed assessment.
Of course, a guided tour might enable you to see more things, but you will probably really only see them. Out of the bus, photo, back into the bus. But if you are the kind of person that likes to look at stuff at their own pace, you are definitely better off going by yourself.
I don't know if you like that kind of thing, but backpacker's hostels usually have very talkative English speaking staff who will give you lots and lots of directions and recommendations if you ask.
I asked the same on flyertalk and the answer was: don't bother with a guided tour.
Though Fifaltra's answer is very good I want to add some stuff as someone who lived in Japan for several years:
1: English announcments are not quite universal. Get outside central Tokyo and they stop. English signage is quite universal around the country however (IIRC there's maybe one private line out in residential west Tokyo that doesn't have it? But unlikely you'd be going there).
But don't worry about this. It's easy to figure out what is Japanese for "next station" and then they say the name of the station.
2: Don't worry about tickets for trains. The way the system works in Japan is if you have the wrong ticket you can just pay the normal price at the end of the journey, no fines or anything. So if run across a station without English on it's big map list of prices (this is not at all common) just buy the cheapest ticket and get on the train, give it to the guy in the window at your destination and pay the difference.
Japanese is pretty easy to pronounce so if you are totally lost even a completely monolingual station worker should be able to point you to the right train if you just say "Shibuya?" to him in a quizzical pointing around you manner.
3: Tours are a big thing in Japan, they often pop up in Japanese media. Going on tours is a very stereotypical thing for retired people to do. Tours aimed at foreigners.... I wouldn't expect anything too great.
Really I'd only advise a tour if you know it's with a particular expert in the site you are going to or it is a very dangerous country. The second doesn't apply to Japan and the first should only be for particular days of your visit, not the entire trip.
4: When I first moved to rural Japan I didn't speak Japanese. After some years there many people also in the same rural area still didn't speak Japanese. In Tokyo people live for decades without bothering to learn Japanese. Japan is a very easy country to get by in English with.
Though Japanese have a stereotype for poor English this is greatly overrated. Most Japanese will understand a lot more English than even they believe they do. In Tokyo you'll never be more than a few meters from an English speaker. In Kyoto and other major tourist areas too, people who can speak English are fairly common.
In Tokyo, you can get around almost everywhere with basic English. Most people will know some English, at least for basic communication. All rail transportation have English signs and announcements. If you have something like Google Maps, you'll be fine (note that Japanese roads usually do not have names; named roads are the anomaly, not the norm, so Google Maps can be tricky from time to time). The first time I visited Tokyo my Japanese was rather rusty, but I still got around quite easily. Unless you want to visit somewhere like a massage parlor, English is almost always fine.
However, many places are not as friendly to non-Japanese speakers. In Osaka, many people didn't speak or even understand English (and I mean people like restaurant waiters and waitresses). In Sapporo, on the other hand, English was fairly well understood. I didn't try to talk in English to anyone in Kamakura, but I don't think people there were that good at English (well, but since it's touristy, I don't know for sure).
In short, if staying in big cities you should be mostly fine; if a store or restaurant doesn't have English-speaking staff, usually try another place. Moreover, all reasonably large JR stations have some signs in English. However, if you go to somewhere more off the beaten track, expect a language barrier. It all depends.
When you take train, be able to tell at least your destination (or, even better, have it also printed down in Japanese). The railway staff will always be able to sell you the ticket and tell the track number.
While many restaurants would even bring English menu for you, there is no any need for it as most of the food is nicely pictured next to the door (very realistic imitations from plastic). Showing with the finger works OK.
Hotel staff usually knows two three English words that are sufficient in addition to the room price printed on the paper. They may have very complex rules on where to wear slippers but just follow that others are doing.
Finally, when once we really needed more extensive communication at bus ticket office, the staff simply made a phone call to somebody speaking English and handed the headset to us.
Most important, stay pleasant and keep smiling regardless what. Also know at least yes, no and thanks in Japanese.