A good basic squash racket is all you need and these are usually quite inexpensive.
Rackets come in a variety of different materials from aluminum at the lower end of the price spectrum, to kevlar, graphite and titanium at the upper end. The stringed area varies considerably but the fundamental rule is: the bigger the head the more powerful the racket. Bigger is not always better; a racket with a smaller head will generate a less powerful stroke but will give you the advantage of more control and will be more manoeuverable.
The type of strings you have also affects squash performance, with more power being generated from the more elastic strings like natural gut, but more control from those with a rougher texture. The weight can also differ considerably, from around 120g (4.2oz) right up to a hefty 185g (6.5oz), which can again affect its control and manoeuverability.
There are four different types of squash ball and are distinguished by a coloured spot, which relates to the speed of the ball whilst it is in play. Yellow spot is super slow, white or green is slow, red is medium and blue spot the fastest.
Beginners tend to use blue (it bounces higher and gives you more time to hit it!) and the professionals use yellow. There is significant variation between the different balls and the first time you use a really slow ball it can feel like you're playing a different sport!
Clothing for squash
As with any sport, specific fitness clothing can be expensive. But for your first forays on court, t-shirt, shorts, socks and trainers are fine. But be aware, most courts won't let you play with black-soled shoes. If after a while you are getting more serious about the game, you can think about investing in a more specific squash kit including clothing that incorporates sweat wicking stretch fabrics and specialized footwear designed to deal with the varied swiveling, twisting and dragging movements of your feet whilst in play.