If you need microphones and the type of microphone you require depends on the situation you're in. Obviously you need a microphone when you want record yourself singing. There's no way to do it without a mic.
When you play the electric guitar or bass guitar you have two possibilities:
- You connect your guitar amplifier or an advanced floor board directly to your audio interface. This is a great solution if you have an amp modeller or a floor board which simulates amplifier models. In other cases you might experience that the typical characteristic of the guitar sound will get lost. In this case I recommend the second solution.
- you connect an instrument mic in front of your amp and record the guitar sound over the mic. This ensures a much better character of your guitar sound.
However, if you need microphones you have to choose which one.
If you want to records vocals, you should get a mic which is suitable for this job. There is an almost legendary microphone for this task: the Sure SM-58. And there are a lot of clones around which are less expensive.
These mics are okay, too. What you should not do is buying the cheapest mic you can find. They will work, but the quality of the vocals will be kind of poor. If you have a low budget, you can try this, but I've warned you: please don't buy toys but only a mic from a music store that says this mic is suitable for vocals.
A question I've often heard is: "Can I use my vocals microphone for recording my guitar"? The answer is: "yes". Although it's better to have a special microphone for this task.
As mentioned above: there's no problem by using your vocals microphone for recording your guitar. There are differences, but they are small. One of the most important differences is that you can bring an instrumental microphone nearer to the speaker of your amp than a vocals microphone. This results in a quite more natural guitar sound.
However, this differences are very little and although they're important when doing some real professional recording in a professional studio, you can ignore this when setting up a home studio - especially when you're budget is not that high.