After you have cleaned and sanded your teak (if needed) you can use teak oil, but that is subject to evaporation and so you have to keep reapplying it to keep up with it. This is the cheapest approach. Sealers like Sikkens Cetol last a great deal longer and is fairly easy to apply. You cannot apply more than one coat per day, but 3 coats are required/recommended (not two) and you have two weeks to apply three coats without having to sand between coats. This will give you a nice matt finish, but if you want high gloss, then you can apply gloss Sikkens over the first 3 coats of the original (you cannot apply gloss alone). The latest version of Cetol has UV filters unlike the previous generations. But like varnish with UV filters, they last only so long and need replenishing (but UV protection is better than those without it. Varnishing is the most labor intensive and difficult to deal with. Teak has natural oils in the wood, so preparation is important for good adhesion. Years ago there were epoxy bases one used to make a barrier between the wood and varnish, but now Epiphanes makes a varnish especially formulated for teak. The first coat is 50/50, the next is 25%, the next is 10% and then you use it full strength, but any good varnish applyer will tell you that a little thinner in varnish will make it flow out better. Read the instructions on the can as it will give you some decent information. To get a real nice varnish finish you have to put on 8 to 10 coats or more. If you sand between coats, then you will remove some, but follow the instructions as there is a window where you can apply more varnish without having to sand. But, if you apply varnish coats too soon, then you won't give the drying solvents to evaporate properly enough and so you can get alligatoring. Read the label instructions.
You don't want glossy wood on surfaces that you walk on. So, if you have teak decks, I don't recommend a gloss. The Sikkens or oil will work well for those. Caprails, handrails, and trim are nice when finished bright (glossy). Inside (I know you didn't ask) you can use a satin varnish which flows out much nicer and is easier to work with than gloss, but they often don't have UV inhibitors in them. Read the labels.
