A first instrument does not need to be expensive, but it should help rather than hinder. A keyboard that cannot play softly, or a guitar that is too big or hard to press, can make early practice a struggle. This checklist covers the features that matter most. Your child's teacher is the best person to check your choice before you buy.

Keyboards and digital pianos

Number of keys. A full piano has 88 keys. Sweetwater's beginner guide describes an 88-key instrument as "ideal for beginners to develop a feel for their instrument", and calls 61-key models "much more space-conscious while still offering plenty of range". Electromarket, a UK retailer, recommends 61 keys when starting lessons and cautions that smaller 49-key models may limit what a child can play. For digital exams, Trinity College London asks that the instrument has enough range for the chosen pieces "without any adjustments to the score". So 88 keys is ideal, and 61 is a sensible minimum for a beginner.

Touch-sensitive keys. These play louder when pressed faster and softer when pressed gently, which is how a child learns dynamics. For its digital (video) exams, Trinity allows electronic keyboards up to Grade 3 only if they are touch-sensitive.

Weighted keys. Weighted keys have some resistance, like the keys of an acoustic piano. ABRSM's piano syllabus allows a digital piano for exams at a private venue only if it has "a touch-sensitive keyboard with full-size weighted keys" and a sustaining pedal, and Trinity asks for a "touch-sensitive, weighted keyboard" on digital pianos in its digital exams.

Full-size keys. Some children's keyboards have smaller keys. Thomann notes that smaller keys are "only partially promising, especially since getting used to the standard size will be the real challenge". Full-size keys are the safer choice.

A sustain pedal. On an acoustic piano, the right-hand pedal raises the dampers so the strings can ring on, as Baylor University's Piano Basics explains. Beginners do not use it at first, but it soon appears in pieces, and Trinity asks for one in its digital exams where the music needs it. Check that a pedal is included or can be plugged in.

Guitars

The right size. Guitars come in sizes such as 1/4, 1/2, 3/4 and full size (4/4). Thomann's age and height guide suggests a 1/2 size for many children aged about 7 to 9 (roughly 120 to 140 cm) and a 3/4 size for about 10 to 13 (roughly 130 to 150 cm), with a full size designed for adults. These are only starting points: Thomann stresses that arm and finger length matter more than age, and describes a simple check: stand the guitar upright, rest your child's elbow on the body and lay their arm along the neck. Ideally their wrist should reach to around the first and second frets.

Nylon strings. For most young beginners, a classical guitar with nylon strings is the gentlest start. Thomann explains that nylon strings are softer than steel strings and have a lower tension, so pressing down is easier.

A good setup. A guitar's action is the height of the strings above the frets. When the action is high, Stringjoy explains, "your fingers have to fight more tension" to make clean notes, which can tire small hands. Stringjoy adds that adjusting the action is "usually simple and inexpensive for experienced guitar techs". Ask the shop to check the setup before you take a guitar home, or ask your child's teacher to look at it at the first lesson.

Tuning. A guitar sounds wrong if it is out of tune, however well your child plays, and most teachers ask students to tune at the start of each practice. Music-Ready's free tuner at free practice tools can help with this at home.

Before you buy

• Ask the teacher first. They may have a view on size, keys or strings for your child.
• Try in person if you can. Normans suggests trying a guitar in person to check it feels comfortable and that everything is within reach.
• Check exam rules if exams are likely, so the instrument suits your child beyond the first year.

A first instrument is not a lifetime decision. Choose something that is comfortable, plays in tune and responds to your child's touch, and you will have given them a fair start.