In 4th edition tau had a preset way of playing and that was mechanized as static armies weren't competitive. 5th edition has reopened the prospect of static with the advent of TLOS.
There are three main, ways to play Tau. Its advisable that you base what you are going to buy on which style you want to play as tau rely on units to support each other. In all choices you require at least a single unit of fire warriors and a battlesuit commander.
The first is static, that is loads of fire warriors + kroot (if you want), sniper drones and broadsides. This army deploys stands and shoots and hopes the enemy doesn't make it. The only mobile component is the devilfish from the pathfinders and maybe the commander and very few suits. If the enemy does make it to your line kroot effectively countercharge. However saying this though I haven't had much experience with this in 5th edition. Just remember fire warriors look good on paper but they just don't stand up to delivering lots of static fire power on their own. Space marines can out shoot fire warrior gunlines which is why the support elements play such a large role through markerlights and "shield commanders".
The second way is mechanized tau which used to be the king of 4th edition, with unkillable skimmers. With TLOS and the skimmer nerf though mechanized has seen less popularity. This army entails everything mounted in devilfishes, using hammerheads and suits to stay out of combat and effectively move 12' per turn. In 4th edition the point of mech was point denial and objective grabbing through unkillable skimmers and unkillable suits which could hide in area terrain. In 5th edition TLOS has prevented true Jump shoot jump while making defensive weapons strength 4 effectively making our tanks either static or slow. Saying this though mechanized tau is still an effective way to play just ask your opponents whether they'll count vechile gundrones as KPs.
The third way is hybrid tau, which uses a fire base and has mobile elements around it. This is the fusion of the two earlier styles and is considered by many the way to play in 5th edition. These armies tend to have a few static fire warrior units, sniper drones, a few mounted ones, suits, pathfinders, hammerheads and broadsides.
There are some units to avoid, however they can be used quite effectively if you know how.
Ethereals are only used for the honour guard fire warriors otherwise you'll see half your army run off the board when they die. They're even less inspiring then space marine commanders which give LD to the whole army while ethereals are only in effect if they are seen by your units.
Vespid, they get 1 shot at 12' which is designed to kill marines. This is good and all except then the marines will rapid fire your toughness 4 5+ save expensive bugs to death, or shoot them with something else after you unload your BS3 payload.
