Tips that may be (or may not be) useful.

Hard to produce high F (on an alto)?

Shade the lower half of the #3 hole (with your left ring finger) in addition to the ordinary 0/145.
This makes the recorder speak the high F easily.
On some recorders, this lowers the pitch a little.

Alternative fingerings for particular sequences.

Rapid alternation of the middle range G with the high E.
Alternate 45 with 0/1245.

Rapid alternation of the middle F with the high F.
Alternate 0/145 with 0/345.

Trio for middle A and G is tricky.
Try using the following fingering.
First A: 0/12345 (very ordinary A)
Next G: 1234567 (thumb hole is fully open)
Tril on #5 hole, that is your right middle finger.

There are many others. So invent your own.

Mute for the recorder

Cut a small strip (3mmx3cm or so) of ordinary paper (copy paper will do).
Put it into the windway from the other side of the beak facing the labium.
You can adjust the loudness by pulling out the strip.

When the paper is wet, it may easily be torn when you want to remove it from the recorder, and a piece of paper may remain in the windway.
When this happens, let it dry, and then blow it off.
Water resistant paper is better to avoid this.
Even better is shiny (partly made of alminium, I think) material used for the container for potato chips and other snacks.
It is durable, perfectly water resistant. And if you want, you can reuse it.

Dropping during ensemble playing.

Tend to get lost while playing?
It is important to catch up when you make a mistake.
If its a 4/4 or 2/4 time signature,
take the first and third notes by tapping with your left foot and the second and forth notes with your right foot.

In preparing for a performance, if you know you always drop at a particular point, or you think particular part of the music is especially tricky, study and mark the position where you know you will be able to come back to ensemble.

Blow off the condensation.

While playing in public, only decent way is to suck in through the windway.
This may not be very wise if you are just practicing because by sucking, you run the risk of sucking in bacteria and fungi in to your lung.
Place your finger across the upper ridge of the windway.
You do not need to cover the window entirely, or stick your finger into the windway.
Never touch the fragile edge of the labium.

Recorder stand.

After playing the recorder, put the joints apart, blow off the condensation thoroughly, wipe the bore and put the 3 joints upright on a recorder stand.
You can make your own recorder stand.
This one is a recorder version of a test tube stand.

Recorder strap.

Place a stopper on a piece of string and put it around the bottom joint.
You either place your right thumb through the end of the string, or

use the neck-strap for soprano sax or clarinet.

Tips for choosing a recorder.

Have 4-5 recorders to choose from.
It is easy to pick one from several candidates.
You may have to make a trip to a big music shop: but it's worth it.

Slowly play every and each note.
Playing fast scales or arpegios does not help: they are just showing off.
Notes of double holes, f# and g# in the alto, are weak notes.
If it is not too difficult to produce these notes, they are just fine.

c# in the alto is an important and frequently used notes; in d minor or a major music.
If this note does not speak easily, you had better avoid the instrument.

Low f and g in the alto are also important.
Usually it is not really possible for the recorder to produce rich and loud low notes.
But you want these notes reasonably rich.

Notes between middle and high c are used most often and important: they should be rich and responsive.
When you blow harder (use more breath), a good instrument will produce richer and louder sound.

High notes should be easy to produce in good instruments.
It is more important for beginners than for advanced players to be able to use these notes easily.
High D is more often used. So I suggest you choose one that produces high D most easily.
If high F is also easy, it is a very good instrument.
High G and E are not problems; they are easily producible on most instruments.
High E flat in the alto is sometimes a little sharp on many instrument.
So do not worry if you think it is out of tune.
You will use alternative fingering to adjust the pitch for this note.

I doubt if a tuner can help.
If you like the sound, a small deviation from the correct pitch is manageable and less of a problem.
You will be able to tell if the instrument is vastly out of tune.
To have an experienced player with you may help.
But after all, it is more important that YOU like the instrument.