Usually,usually but not always but more times than not when you read a tab and it says something like 3-5-7 usually your index finger (your pointer) will come down on the first number/fret (3) and in this case if 5 is the next number then your 4th finger (the one in between your middle finger and pinky) will come down on the 5th fret and your pinky will come down on the 7th fret. A fret is the square blocks in front of and behind the fret posts or metal bars running the length of the neck on your guitar,some guitars have 22 frets some have 24 and some have even more,at the top of the guitar (towards the headstock where your tuning keys are) the "frets" are larger spaced,at the bottom,(towards the body of your guitar,your pick ups and tone controls the "frets" are much,much smaller. A good technique to use to practice finger placement is the "spider technique" it uses all of your fingers in the exercise while playing all of the different strings and you can use it up and down and anywhere on the fretboard,it takes time and much practice but after you get it down picking it with downstrokes and then practicing it with upstrokes you can then take it to the next level by using alternate picking (using both up and down strokes,up on one string/fret and then down on the next fret or string.) Tabs look like this,each line is a string on the guitar I only mentioned the high e and low e strings because there are so many different tuning styles that it can get complicated trying to figure out which string is which,so for now the low e (the thickest string on your guitar) is the starting point for most scale tabliture and the high e (the thinnest string on your guitar) is the ending point

------------ high e
------------
------------
------------
------------
------------ low e


here is the "spider technique" that I used to help me with different picking styles and proper finger placement as well as being a great exercise for speed.Starting on the low e (the thickest string on your guitar) the first finger to hit the 2nd fret (2) will be your middle finger,the next finger to hit the first fret (1) will be your pointer/index finger,the next finger to hit the 4th fret on the string (4) will be your pinky finger,the next finger to hit the 3rd fret (3) on the string will be the finger in between the middle finger and the pinky finger,after playing the last fret on the first string move down to the next string starting again with the middle finger and do the same thing that you did on the first string on the second and so forth and so on.First practice using down strokes,then do the exercise using just upstrokes then use alternate picking.Practice this technique for at least an hour a day and before you now it your picking speed and finger dexterity will increase greatly.


-----------------------------------------2-1-4-3-
---------------------------------2-1-4-3-
-------------------------2-1-4-3-
-----------------2-1-4-3-
---------2-1-4-3-
-2-1-4-3-

another exercise is just like this one but using just the last two strings the high e and the one above it or any two strings for that matter but if you use it on the last two strings it helps you to become more comfortable stretching your hand to play on those strings and it helps you become more comfortable playing the strings most guitarists incorporate to one degree or another for their solos.You can also move your fingers to the next string after the two I show you as you did in the above exercise,it's almost the identical exercise but with a different starting position and in reverse if you take it from the high e to the low e.Start on the high e (thinnest string) and hit the 4th fret (4) with your pinky finger,then hit the 3rd fret (3) with your ring finger (the one in between your middle finger and pinky finger) then skip to the next string and hit the 2nd fret (2) with your middle finger and then hit the 1st fret (1) with your pointer finger,then transition back to the high e string and using your middle finger and hit the 2nd fret (2) followed by your pointer finger coming down on the 1st fret (1),then transition back to the next string above the e string and use your pinky on the 4th fret (4) and your ring finger to come down on the 3rd (3) fret.And like I said you can take this execise all the way up to the high e string by just repeating the exercise on the following string above the one you ended at

4-3---2-1----
---2-1---4-3--4-3---2-1
------------------2-1---4-3--4-3---2-1
---------------------------------2-1---4-3--4-3---2-1
------------------------------------------------2-1---4-3--4-3---2-1
---------------------------------------------------------------2-1---4-3

again first try it with just downstrokes and then with upstrokes and then with alternate picking after you feel comfortable with the other picking styles,do this exercise as well for an hour and if you stay vigilant,practicing these exercises for an hour for one month's time your finger dexterity,picking techniques and picking speed will drastically increase,you'll be on your way to becoming a guitar player.