I have taught guitar for about 30 years. I was once a beginner. When I teach, I remember what it took to get better and try to convey that to my students. The following works for guitar players and probably most instruments as well.
Playing an instrument is at first, a physical problem and NOT a musical problem. You must solve the physical problem before you can address the actual music making. When you play guitar, what is it that you really do? You hold down the strings with one hand and strike the strings with the other. That is it! Not too romantic is it?
When we play basic songs on the guitar, we will use chord shapes that we change between. Each chord has a unique shape. A basic song may be 4 strums each of a G, C, G, and D chord. Not until you can change chords quickly can you actually play the song. Solve the physical problem! Simply change between G and C chord 714 times. Don't strum, just change chords. If you can do it without looking, you have it. Then conquer C to D by changing them 687 times. When I teach young people, I will tell them to "grab G and then go to C". I time them for one minute and count each time the C chord is placed. After 4 or 5 minutes, we see the numbers grow as the physical problem is addressed. It also makes it a game! After a while I can see their score and tell them "see, you have practiced for 7 minutes and have gotten 5 times faster".
Do you really have to change chords 678 times? The number is random but the point is that there is a finite number of times of changing chords it takes to conquer it. If you take a month to change 678 times, you may never stick with it. The point is that you can solve the problem NOW!
Once you have conquered the chord changes, you can play basic songs. To play a song we generally use a pick and strum the strings. When you start, use a thin or medium thickness pick. You will be holding on to the pick firmly as you fight to not drop it. The pick must be flexible.
To play a basic song, remember the downbeat is the key. Don't at first worry about coming up with a fancy strum. Any varied strum will always have the downbeat within it. Start with the downbeat and count. You must be able to strum in time and change chords without slowing the beat or stopping. There's that pesky physical problem!
Get an "Easy Guitar" songbook. You will see the chord shapes printed above the staff. Learn to play songs YOU like.
If you have questions, feel free to call me. Better yet, come over and let us teach you!
John Walsh