My second hand Hercules bicycle came with 10 gears. Two gears in the front and five in the back. The gearshift does not work well and since I cycle in the same gear all the time anyway, I consider turning it into a single-speed bicycle. Can I just remove the derailleur in the back and adjust the length of the chain? Any reason why I shouldn't do this besides the obvious fact that I will have less gears? Anything I would have to take care of?
I've got a 2nd hand Hercules too and I've done this exact thing. Get a quality chain tool and look up some videos on Youtube about how to remove and re-link a bike chain. I pretty much winged it as I had no clue what I was doing. It took a bit of work and a lot of frustration to get the chain length right. I probably did it wrong, but it works.
If your bike has slotted dropouts and a rear wheel secured with axle nuts (which Poster's answer implies you have) you can do a single speed conversion without the need for a chain tensioner device.
The derailleur can be removed, chain shortened and tensioned properly by adjusting the position of the rear wheel. The chain should be able to move 0.5 inch up or down at the halfway point between the sprockets.
Link to Park Tool video on single speed chain replacement and tensioning below.
You can use the existing chainrings and cassette like Poster did but you need to consider what gear ratio you want and what sprocket/chainring combination you will to use to get that. You ideally want a combination that allows the chain to run as straight as possible.
You probably want to get a new chain rather than shortening the existing one as it's more than likely worn out. Check how worn the cassette sprockets are too.
Any bike can be converted to singlespeed, but you'll need a bit more than just cutting chain to length. Chains will stretch over time and require periodic tensioning to ensure they are correctly aligning with the cog and chainring. This can be accomplished with tensioning bolts through the dropout (only some frames come equipped this way) or using a chain tensioner that attaches to the same point as the derailleur but only contains a single jockey wheel with a spring that keeps the chain from going slack. A number of companies make them and all work in generally the same fashion. Surly singleator, The dangleberry... etc. Just do a web search for bicycle chain tensioner and you'll get many hits.
Just be sure to pick a gear ratio that works for your geography and desired level of effort. The most common gear ratio on modern singlespeeds is 42:16 if you want a starting point.
Edit to add: The type of dropouts on the bike will also have an effect on achieving proper tension. Older road frames will often be equipped with a small set screw through the dropouts on one or both sides to add/release chain tension. More modern/dedicated singlespeed bikes will utilize horizontal dropouts (really just a sliding track for the skewer/rear axle) and may or may not employ the set screws for tensioning. In those cases, tension can be achieved manually by pulling the rear wheel into tension but I'd caution that, without some type of set screw on each side; it is very easy to misalign the wheel in the rear triangle and/or pop it out of tension under hard pedaling efforts.
As @Poster's answer already shows, it is very doable, it is generally reliable and the only tools needed are a chain tool and an appropriate wrench for the rear axle nuts.
I'd say that there are 2 levels of involvement regarding this conversion, at least from a budget and time perspective.
Low budget, low effort
One reason to do this conversion is having no budget for repairs, or not being willing to spend money on that bike (it is a beater commuter, for example) or not having all the resources at hand (an emergency)
This option involves only removing the derailleur, shortening the chain and adjusting chain tension by changing the position of rear axle in the dropout. This applies to horizontal dropout frames only.
For vertical dropout frames, you may need to keep the derailleur. Remove the cables and use the regulator screws to keep the derailleur at a specific position, so it acts as a chain tensioner.
This setup works fine, but you may have trouble with chain dropping, and the bike will still have parts that are not being used, you'd be carrying that weight for no reason.
The solution? Changing the rear cassette for a single speed freewheel and use a single speed chain. If the chainrings are removable, remove all the unused ones. If possible, install a singlespeed chainring.
This leaves the bike with a cleaner look, it will be also, easier to clean. (pun somewhat intended), it will be lighter and more importantly, singlespeed parts are beefier so they take a bit longer to wear out.
Singlespeed chains are not designed to bend side to side, so the links are thicker. The whole chain is thicker also, as it doesn't have to fit between tightly spaced gears.
Multispeed chainrings and cogs may have ramps, bumps and other modifications to help ease gear shifts, which may be counter productive in a single speed setup. On the other hand, single speed specific parts have no such features at all when they are the cheapest options, they are literally "plain" gears, while higher quality components may even have chain retention features, all of that favoring a less chain-drop prone ride.
Changing a freewheel however requires special tools, depending on the particular freewheel. Once unscrewed from the hub it's only matter of screwing in the new one. For freehub equipped bikes, a single-speed conversion may be necessary, It consists of the single cog and some spacers, however, a freehub is more common on modern bikes with more than 7 cogs.
Te same goes for removing the chainrings, as a crank arm puller may be necessary. The chainring themselves are usually bolted to the crank spider for which you may need a hex key.
Some cranksets bolt two chainrings with the same bolts, so in order to only remove one of them, you'll need either shorter bolts or spacers (washers the same thickness as one chainring) When replacing the chainring for a single-speed specific one, it may already be thicker than the multi-speed ones, so the washers are not required.
In case the bike has a single-piece crankset, the only option may be changing the hole part. Another possibility is that the bike has cranks that can be separated from the axle, but the chainrings are welded or riveted to the spider. There the options may be changing the crankset, or leave the one that exists as is.
For vertical dropout frames, you'll need to add a proper chain tensioner, either bought readymade or DIY
For horizontal dropout frames you may want to add tension regulators, these are additional bolts and pieces to keep the axle from sliding along the dropout, making the setup more reliable.
All these modifications are not strictly necessary, and not necessarily have to be done all at once, with the exception of the singlespeed freewheel or cog and chain, as the difference in thickness/width may not allow it. However, a singlespeed chain works good enough with most cheap or old multispeed chainrings.
The good thing about all of this is that singlespeed parts are very basic and easy to manufacture so they are available in a very wide price range. Last year (2017) I restored a singlespeed bike and the freewheel and chain did cost about 12 US$. Sure, the parts where from China or Taiwan (I don't remember exactly) but I was pretty confident that they would last enough.