If you do Space Wolves, Bikes are more of a viable strategy than if you go basic Space Marines, I'll give you a quick run down, but the best way to find out what you'd like is to go down to a Games Workshop or some local hobby store where people play and just kind of look around and see how the armies are played, and what the models look like
Space Marines: Very resilient, very balanced army, basic infantry have good shooting and good armor. Their elite choices can get quite deadly, for a cost. Their tanks can either be balanced out to be pretty good at both anti-infantry and anti-armor, or specialized to be really good at either, for pretty cheap. The trade off is, they have a relatively weak armor value.
The different chapter codices specialize in various things, it's definitely worth it to check them out
Imperial Guard: Infantry. Lots and lots of Infantry. For every squad of guardsmen that fall there's 2 squads to take their place. weak infantry, but can dish out a horrendous amount of firepower. There basic weapon is the weakest in the game, and their statlines suck, but you can output so much firepower that you can often force the enemy to take so many armor saves that the start failing. Plus you can outfit heavy weapons in infantry squads, and take dedicated heavy weapon squads, which can be devastating at anti-armor or anti-infantry. Veterans are good, they can be highly mobile, can get better armor, plus can get 3 special weapons, which can be really good. Officers can be deadly, and can issue orders every turn to infantry squads to augment their abilities, such as +1 shot to their lasguns, or twin linking weapons that are firing against vehicles or monstrous creatures, which means they can reroll the first dice result. The other half of the Guard is Tanks. Their basic tank is the best tank in the game, and every variant is devastatingly good at what its specialized in. Their artillery is awesome, but only truly awesome in higher pts game.
Orks: WAAAAAAAAAAAGGGGGGGGGGGGGGGGGGHHHHHHHHHHHHHHHHHHHHHH Their armor sucks, but have the numbers to make up for it. Their whole point is to get into close combat, and every aspect of the army from their basic infantry to their elites are terrifyingly good at it. Stormboyz and Bikes can close in quick, and still be ridiculous good.
Chaos Space Marines: Like Space Marines, but can take awesome options, plus the fluff is awesome. Their terminators are the cheapest base wise, and their Obliterators are just insane.
Grey Knights/Inquisition: Highly Specialized, over the top badassness that will pretty much be outnumbered all the time and be the most hated army on the table lol, so you gotta have balls to play them.
Necrons: The new codex kinda changed it around so they have more options than just "walk the squads behind the monoliths", not too familiar with the new codex, but their infantry are pretty solid and have some pretty cool support options.
Sisters of Battle: Kinda hard to describe, not quite Space Marines, and have different options
Eldar: Highly mobile, relatively weak infantry, with fast tanks that can hit hard, and cool psychic abilities.
Dark Eldar: Kind of the same thing, with different options
Tau: Very Shooty Army, with longer range guns, and their anti-tank guns are terrifyingly effective because they are such long range and high strength, plus can put anti-infantry rounds also. Their battle suits are highly mobile and can get a lot of special weapons.
Thats the basic rundown, if you have any questions quote me or PM me, I've been playing for a long long time and am happy to help new people starting out wherever I find them. It's a great hobby, and the most important thing whether you win or lose is to have fun.
EDIT: forgot the Tyranids: Either they can swarm the enemy or go big scary monsters that wreck everything
