Voice strains very quickly

Hello -- I've been learning to play piano and now find I want to sing along as I play, so I'm trying out these online lessons. However, I'm in my 60s and haven't done any real singing in, well, decades. Even in Christmas Carol sing-alongs I find that my voice strains and starts cracking quite quickly. Same in the warmup lessons here that I'm just starting out with. A couple of rounds of a simple warm-up and I'm already feeling the strain in my throat. 

Is this common when starting out? Is it a problem with technique? I have a feeling I'm "forcing" the voice rather than letting it flow, but don't know how to fix it! 

Hi, Sandy! It's common to feel some strain if you're trying to stay in chest voice the whole time. I encourage you to let your voice "lighten" or "flip" into head voice when it needs to! Singing on something closed like a lip trill can help you find that coordination. 

Please see below for more guidance on finding your "mixed' coordination from chest voice:

First, make sure you're exercising your head voice! Those head voice muscles (CT) need to be working. Use a full-range lip trill or OO to let your voice "hand over" from chest to head. As you train your chesty mix, that "hand over" will just be more gradual, with less head voice.

In that middle/mix range where you're starting to feel strained, try using a tall + narrow mouth position (OH or UH shape, rather than a wide HEY or YEAH shape) to encourage more mix (head resonance along with chest). This will relieve some pressure and help you avoid the unstable "shouty" pulled-chest voice sound.

Here's a video with a singer using that kind of "tall and narrow" vowel modification: https://www.youtube.com/watch?v=BVqS_BYsDBA. Listen carefully to the tone after he modifies the vowel. It goes from shouty chest voice to a strong but more balanced "mix". 

Here's an article on the kind of vowel substitution he used as well: https://www.thenakedvocalist.com/quick-fix-vowel-substitution/.

These are the 3 exercises I like best for stretching chesty mix: https://www.youtube.com/watch?v=qwiwZ3dS8mg&t=408s

Here's a short lesson on avoiding "flipping" within mix: https://www.youtube.com/watch?v=6pI9v1YMFN4

If you're getting a shouty tone, here's another lesson addressing problems with mix: https://www.youtube.com/watch?v=vzObbNv78oA.

I also love this teacher's demonstration & explanation of mix: https://www.youtube.com/watch?v=pYsO7HX84P8. 

Lastly, remember that you're building coordination (between your TA and CT muscles) while you stretch (CT) and continue to resist air (TA). Progress might be slow, but keep at it!