I'm having a bit of trouble in tuning. I've tried tuning apps and even bought a tuner. However, when I pick the E string, it says it's the B string; when I pick the G string it says it's the D string. Please help!
First, replace the strings
Old strings will not, ever, stay in tune. They physically can't, because of the way they degrade over time. Strings over a year old must always be changed, even if no-one touched the instrument in that time. (Instruments played regularly, you might only get a month out of a set. Pro musicians might put a new set on for every gig. But a year is the outside limit for the longest time.)
If you can't do it yourself, get a music shop to do it. They'll charge some nominal amount, and then you'll know it's all OK.
Damp the other strings during tuning
When you pick one string, the vibrations through the instrument bridge will excite all the other strings, making them ring too. The adjacent strings are closest on the bridge, so will be most affected. Tuners are very bad at dealing with extra notes happening at the same time, so this is a real problem for them. There's even more problems when it comes to strings ringing at other harmonics too.
The normal way of tuning using a tuner is to hold your fingers or the palm of your hand over the next-door strings as you pick the one you care about. You should then be guaranteed to only get the string you want.
Perhaps it's a baritone guitar?
Baritone guitars are naturally tuned down a 4th from standard tuning - B-E-A-D-F#-B. A proper baritone guitar will generally have a longer scale length (neck) and a slightly different bridge setup to deal with this, but you can make it work on a regular guitar with thicker strings and less tension.
Baritone guitars are fairly rare. Guitars with random stringing are not, and too much tension can seriously damage an instrument. New strings as per the first step solves both problems
Some tuning apps or outboard tuners have a transpose function which may have been inadvertently activated because all your intervals are off by a Perfect 4th. However, if you are not familiar with how a tuned guitar should sound then you may be off by that amount. Try comparing your guitar‘s pitch to this or a similar video to see if you’re in the ballpark first. Good luck!
It sounds like you're new to the guitar, and there are a variety of things that could be causing the problem. Your best bet will be to go to someone you know who has guitar experience, or take the guitar to a music or guitar store, and ask them to check it for you: to make sure the instrument is set up correctly, that your tuner is working, and to help you tune it for your first time.
One possible explanation is that you have a 7 string guitar. Most 7-strings have the lowest string tuned to B, and the remaining 6 strings tuned as a regular guitar. When you say all strings are off by a fourth (which is the tuning interval between guitar strings), this sounds like it's a possibility.
So if the guitar does have 7 strings, you should expect the second string to be tuned to E, not the first.